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.
All the sisters speak English with no trace of an accent. Only her mother has any difficulty with the language, and that's because she has always worked with other Vietnamese refugees.