Treat him normally. Always let him finish sentences even if you know what he's gonna say and don't ever rush on him even if you're in a rush for something. Reaching somewhere a minute or two late would be better than cutting him off mid sentence and letting the fear of talking build up in him. I know u can't get the students in his school to do these things but do take a notice if he's being bullied. That makes it alot worse. And TAKE HIM TO A SPEECH THERAPIST RN. Everything aside though, you seem like a pretty loving parent i wish i had :3. Stammer is kind of in my genes, my father, an uncle and my brother had it. They all recovered by themselves around the age of 14-16. So they never took me to a therapist or anything. Was in a public school where teachers beat us a little too(actually pretty normal in government schools here) and got bullied alot as well. Maybe thats the reason that I'm almost graduating university now and my stammer is worse than it had ever been. Worsening every year. And i wouldn't wish it on my worst enemy. I just wish ur son is treated better and get the attention that he needs. (Sorry for English mistakes, if any. Its not my first language).