Bullies like the basketball player thrive on intimidation. If this boy gets away with physically attacking once, I can guarantee he will do it again. The school most probably can't (and won') deal with an attack off the school grounds, but you as parents can file assault charges against the boy with the local police, especially if there are any further threats or attacks. At the very least, I encourage you to talk with the student resource officer if your school has one. Do whatever you need to protect your son from retaliation from the other boy.
Your son may not want you to interfere, but unless he comes up with a plan of his own, with which you agree, I encourage you to hold your ground and be his parents.