Sorry to hear about the bullying. Truth is its a fact of life. Good supervision can minimize it, but elimination is not possible. We had the same problem when my son was young. He was small for his age, skinny, smart, not very athletic and not very aggressive. To forestall bullying, I enrolled him in a serious, traditional Shotukon Karate school. we also gave him the same insructions you have given your boy. After about six months of instruction he had rudimentary self- defense skills. After a year, he could hold his own. One or two incidents of bloody nosing the bullies made the bully crowd move on to easier prey. In the end, Karate became his sport. He studied it for 12 years, competed all the way up through international level tournaments. He never changed his gentle approach to life, even though, on a few occasions, he had to deal with bullies. He will tell you that Karate changed his life for the better, in all aspects. It proved to him that if he applied himself to the task, even if it was not easy for him, he could generally prevail. A great lesson to learn in one's youth.