WOW, since your husband now knows where the bully is learning, it would not be an unusual situation for him to wonder off to that yeshiva and have a one on one with him. It could be a beneficial conversation for both parties. Your husband as the father of the one who was damaged and the boy himself who had decided to move up and take life a little more seriously. This boy has an obligation to do a bit of “tikun” and has an obligation to ask mechilah. Your husband can offer him the opportunity. He can suggest to this boy who is a bit more mature than he was at the time, that if he so chooses he can a write a letter to your son at “your address” asking mechila from him and explaining to him how wrong he was for doing what he did. Your husband does NOT have to tell him everything that is happening to your son other than his rishus hurt him very badly and no one knows how deeply ones actions can hurt another nor how long lasting or far reaching those hurts can manifest. On the other hand, no one knows the true healing powers of real teshuva and how long lasting and far reaching that can manifest either.
On the other hand, no one knows how far reaching and long lasting the effects of NOT asking mechila and not doing teshuva for ones actions can manifest either. This my friend is a well known fact and a true dilemma. Your husband would be doing this young man a favor, helping him to clear this up early in his life.