(ooops, sent it to myself, here it is again to you)
Just speculating here but:
If he was 41 then he was born in 1968 in Vietnam. If his family fled towards the end of the war (1975) he was probably about 7. From personal experience, many refugees from Vietnam did not become citizens until 5 years after arrival (1980) which would come close to the timeline that he was a citizen for 28 years.
Your idea, if true, that he was still going to English classes after all these years is interesting. That would mean he would have isolated himself in just the type of “Balkanized” country we don’t want to be.
The timelines are almost identical to my wifes (also a refugee from Vietnam) but she grew up in a regular community and speaks English with no accent at all. A few of her relatives that live in California have so isolated themselves that they can not function in an English only environment. The ones who have adopted pure English are all successful.
Could also be Hmong, and have come here in just the last few years. Just speculation here, too.