I would encourage you to find other children who stutter so that he can see that it's not a big deal. There are a lot of great stuttering organizations out there (I don't know if you are in the USA or abroad). Also, don't make it a big deal and most importantly, let him finish his sentences. I'm not going to lie to you, school kids can be cruel 😔 and when he is teased, be that support and listen. I hope that helped.