As a children’s mental health professional who also stutters, I would think that teaching your child to advocate for himself would have been the very first thing you and his assigned SLP would do. Upon introducing himself say “Hi! I’m Johnny. Sometimes my words get stuck. It’s just how I am! Hey, do you want to play tag?” No apologies, keep it simple, and then move on to something fun. The other children aren’t really asking for the science about why his words get stuck. That are just pointing out something they notice, which is completely natural (and is likely coming from an emotionally neutral place) at that developmental stage. What exactly is the SLP’s training?