Is your son already one of the older ones in his grade, by virtue of his birth month? It can become very frustrating for a young man who is already 18 before even starting senior year, to comply with the various rules and restrictions imposed upon high school students (things like freedom to come and go from campus, engage in sexual activity). Honestly, it’s hard for all of them to comply with the rules in senior year, let alone for a young man who is a year older than the rest.
Also, after four years in an English speaking school, if he had been reading for pleasure outside of school, his English would have caught up to that of his peers. This is what I tell children of immigrant parents when the parents do NOT speak English well. I encourage them to read, read, read in English, so that they are exposed to the full level of English vocabulary. In fact, that’s what high-achieving American students do - they read a lot of literature to develop their vocabulary and writing styles.
This is really your son’s decision. I would most definitely NOT have him repeat at his own school. If he agrees, I’d move him to a private school to repeat 11th.