An American Expat

M cames from Florida and came to China nearly seven years ago.

Your version is OK, but this way you can avoid the "come" repetition.

When he studied biology at the University of Florida, he got to know a Chinese visiting professor, through whose connections he went on to attend a grad school in my city and became the first American to get a master degree in science in at that university.

To be honest, his lectures have always been very informative and helpful, but since he only speaks English during the entire class, which is difficult for most students find it difficult to understand him, and thus the attendees keep falling in numbers, though I’ve noticed that he’s already tried to make his English simple as much as he can.

Your original sentence ("which is difficult...understand") could also mean that the class itself is objectively hard, not necessarily the language.

"Attendees keep falling" sounds like they trip and fall down.

I heard him speak Mandarin once after class but it wasn’t fluent, so I guess that was why he didn’t speak one bit of Mandarin at/during the lectures to explain things.

M said he returns to America twice a year and wanted to spend half a year staying in the US and another half in China, though he’s spent most of the time in China so far.

"Staying" and "spend" repeat.

American expats like M are more or less uncharacteristic: they’re either bored with (their) American lifestyles or are simply fascinated by the culture here.

A Floridian, M came to China nearly seven years ago.

Another alternative.

When he studied biology at the University of Florida, he got to know a [Chinese visiting professor / visiting professor from China], through whose connections he went on to attend a grad school in my city and became the first American to get a master's degree in science at/from that university.

He still works in the field of biology and is running a start-up that mainly deals with attracting American technologies to China and looking for Chinese investments / investors to commercialize them.

He also volunteers, and has taught English at the city library every Saturday since last November.

To me, "last November" would be "November of last year (2016)." Since "今年11" was just a month ago, I would simply say "since November."

I [went / have been going] to the library every other weekend so I happened to attend his first lesson.

M’s [a Caucasian and looks good / a good-looking Caucasian]; the library advertised his course in this way: “A handsome American CEO, an Ivy League university graduate, teaches English here!,” in an attempt to attract as many students as they could.

Ha, ha! The University of Florida is by no means "Ivy League." :-)

To be honest, his lectures have always been very informative and helpful, but since he conducts the class entirely in English, which is difficult for most students to understand, the attendances keeps falling, though I’ve noticed that he’s already tried to make his English as simple as he can / as possible.

Despite growing up in the American South, M speaks “standard American English” without any trace of a Southern accent, which was new to me, because I’d previously met American Southerners who spoke heavily accented English.

Northern Florida, which is very rural and similar to Alabama & Georgia, is definitely part of the South; many natives speak with a Southern accent. But southern Florida is quite cosmopolitan, with many residents having migrated there from the Northeast or from outside the US, so the local accent is pretty much "standard American English."

He created a WeChat group, where he commented / comments on students’ recordings and corrected / corrects their pronunciation, but now he is taking a break because he’s heading back to the US for Christmas.

M said he returns to America twice a year and wants to spend half of each year (staying) in the US and the other half in China, though he’s spent most of his time in China so far.

By the way, M is engaged to a Chinese lady from a rural area.

You could say he "has become engaged," but "is engaged" is simpler.

American expats like M are more or less unrepresentative: they’re either bored with American lifestyles or are simply fascinated by the culture here.

"Atypical" is the right word, but I see you've already used it in the next sentence. "Abnormal" would work well. :-)

I can sense that M’s an atypical as an American in some ways, though I cannot put my finger on it.

"M's atypical American" sounds like "M's" is a possessive, not a contraction. If you change it to "M is atypical as an American" it will be OK.

Anyways, being an expat for so long isn’t easy, and what I can help out, to say the least, is to cooperate in his class. / ...so the least I can do is (to) support his classes by participating enthusiastically.

"Help out" sounds odd to me, but "支持" sounds fine. And "cooperate in his class" sounds like the alternative might be to cause a disturbance.

COMMENT: In English, whatever come at the start of a sentence becomes the most important thing. Your original sentence is possible *if* the fact that _you_ met this person is the most important thing. One politeness strategy (seldom taught) is to avoid placing "I" at the head - it makes the writing seem egocentric. Here is one alternative wording - About a month ago I came to know a young American guy named M through an English language course.

He said that he fell in love with this country, so (he) decided to stay here after graduation/graduating.

COMMENT: Both the original and the correction above are possible, but the phrase "to commercialize them" should probably be deleted. If you are seeking investments or investors, readers can easily infer that this is in order to commercialize/market an item.

COMMENT: Both the original and the correction above are possible.

COMMENT: The correction above is possible, but culturally I found it odd. Race should not be a factor in academic learning. Whether the person is white or black or yellow or whatever is irrelevant. And being "handsome" might be important for marriage or romance, but should it matter when selecting a teacher?

Here in Japan, many conversation schools try to market "handsome" or "beautiful" young teachers to the public. It makes me think deeply about what teaching actually involves. At first, some kind of superficial emotional attraction might be important. In high school I had a beautiful German teacher. Her beautiful breasts kept me alert during class. Her alluring smile was something that almost every male student yearned for.

SUGGESTION: The text would flow more smoothly if the long sentence were broken into several shorter sentences. Here is one option - To be honest, his lectures have always been (very) informative and helpful. However, since he only speaks English during the entire class, many students are having difficulties. Attendance has been falling, though (I’ve noticed that) lately he’s been trying to make his English as simple as possible.

I heard him speak Mandarin once after class, but it wasn’t fluent. It seems that he is unable to explain things in Chinese.

NOTE: I reworded to use the word "I" less, a politeness strategy.

LOGIC: Ironic quotations are not needed around the word “Standard American English” - unless there is some satire or irony implied. Despite growing up in the American South, he speaks “Standard American English” without a(ny) trace of a Southern accent. This surprised me since I’ve met other Southerners with thick accents.

According to Wikipedia, Standard American English is a proper noun [Cf. https://en.wikipedia.org/wiki/General_American]

He created a WeChat group in which he comments on students’ recordings and corrects their pronunciation. However, now he is taking a break because/since he’s heading back to the US for Christmas.

CULTURAL POINT: If he is Christian, the term "Christmas" is fine. If he has no religion, then
"winter holiday" is a better word choice.

COMMENT: In terms of grammar, I think Correction #2 is fine. Personally, I believe it this way: the USA has only 6% of the world's population. China has 20% of the world's population. India now has 17% of the world's population and may become even bigger than China. I am not bored with the USA - but fascinated with the world. (The USA is only a very small part of the world, and our world is only a very small part of the universe. We should be fascinated with the entire universe.)