I'm less concerned about the jackholes making assumptions than you making sure your son knows that he's done nothing wrong, and that some people are just judgmental cretins. At his age, he's going to take everything too personally anyway. Maybe instead of addressing the yeller, you can look your son in the eye and say, "Sometimes, grown ups are ignorant and don't understand that kids can have physical challenges too. The best thing to do is ignore those people." (But just to him, not in a too-loud way as a backhanded insult to the person making a scene.)