hi kodokan...
if you're worried, there may be a reason, trust your instincts...
my son joined swiss german kindergarten in august, we've been here since march, english is his second language, we speak afrikaans at home. so language is a HUGE issue to me, particularly because performance in primary school is crucial due to the way the school system works here and also because we can't help him...and of course, being a true SA male - he doesn't speak about things he does in any case, so i don't know what's cooking!!!
shamefully i can't offer you a solution, except also to talk to the teacher regularly and then follow up on the feedback. if you feel he has to play more in french, fetch the friends yourself.
i was cautioned not to make it too big a deal, because he could pickup that i'm worried. but on the other hand if you sit back and let the system take its course, he could be the loser and you know about it too late?!
wish i could be more helpful, but really, follow your instincts, you're his mom! you know best and have xray eyes and bionic ears and ESP in your heart...