With kids, especially at that age, it is going to happen. I remember at his age, (Im 54) and the teasing just happens. Not sure what the right answer is, but I always stood up for myself. I got in more than one kids face over it. It is a fact of life for those of us who stutter. There will always be people to meet who do not understand what stuttering is, why we do it, and unfortunately it does make us an easy target for others. Like I said, I always defended myself. That and found a group of friends to hang around with. My friends were never ever bothered by how I talked. They defended me, and I defended them.