I think it's wonderful you care for your son so much and are seeking advice on how to help him best. Just knowing you care will help him so much.
I think it's good to discuss these incidents that upset your son, but encourage a quick resolution. Going over it and over it will just encourage your son to obsess and worry over the incident more. You might just say something like: I'm sorry that boy was mean to you. Had you been mean to him? No? Then I guess he's just a silly boy. You did nothing wrong. ...Then just go about your normal routine with him. It's important for your son to know that things still go on in spite of what happened to him. And that it's okay for them to go on. The less his life changes as a result of the abuse, the easier it may be for your son to learn to put it in the past.
It's a slow process. Have patience with your son and with yourself as well.