First of all, Twisted Sausage makes a good point. Is he even worthy of being forgiven for? For you, yes it would be easier to get it of your chest and be able to let it go. You have no way of deciding what choices he makes, but you can decide how you react to it, deal with the effects of his actions. I don't know what happened between you two, but holding on to something like that will only drag you down with it as well.
I'd like to leave you with a quote by the awesome Mandela, “Resentment is like drinking poison and then hoping it will kill your enemies.”