Self-forgiveness can be more difficult than forgiving other people because we tend to be harder on ourselves than on others. So, I would recommend that you first approach your son with the idea of forgiving someone who has hurt him. Let him get used to the idea of offering goodness toward at least one other person. Then try it again with yet another person. Once he sees that he can offer goodness toward others who have hurt him, then ask him to consider offering this same goodness to himself: unconditionally and compassionately. If he sees the need then to seek forgiveness from those he has disappointed, he could do that.