Was an ESL teacher for 20 years, and also a citizenship teacher. If he has lived here long enough to become a citizen ( and passed the test), he should have a decent command already. If he has just learned English by being around others that are also Korean, he will probably continue to speak with errors, but that shouldn’t stop native speakers from understanding him.
OTOH,his 25 year old daughter...who he brought here as a kid...has a command of English every bit as good as mine and just graduated from Columbia last year.
Native speakers can understand him,with difficulty,but the issue bothers him greatly.