You can't deal with this for him, I mean maybe you can talk to his teachers, counselor or speech therapist and they can watch for it, but that's a short term solution. Help him understand that his stammer doesn't change anything about him, that people are different and have different strengths and weaknesses that they can't always control. Kids will always make fun of differences because it's easy to identify when someone is different. My parents never shamed me, tried to change me, or made me feel bad about my stutter. They encouraged speech therapy, which never did much, but I never felt like I should feel bad about my stutter. I've never taken insults too personally because I know that my stutter doesn't make me a bad person or change who I am in anyway. I'm def more empathetic, but I think that can be said of anyone who grows up with a disablilty. It still feels true today, the less I cared about my stutter, the less everyone else seemed to as well. Good luck to you and your son! Tell him to keep his chin up, and take his time. Teasing someone about a disability says a lot more about the person doing the teasing.