I think you need to be firmer with the teachers - tell them he is telling you he is not being given any choice about whether to play with this boy or not, and would rather play in a larger group. See if you can get your son to give you a direct quote you can go in with.
My 5 yr old son is being emotionally blackmailed into being friends with a boy in parallel Yr 1 class - my son keeps telling me he has to play with this boy or he'll get angry with him & as a result has drifted away from a really lovely group of friends he had before. I've tried talking to teacher but they say they have observed them at lunch break & they play nicely together but I think this is because my son is worried not to because he is very mild mannered & doesn't want this other boy to have a go at him. I've tried explaining to my son that he shouldn't have to do things he doesn't want to but he doesn't seem able to really understand this. I'd really appreciate any advice from people who have been in similar situations ?