Again, I'd ask other outlets instead of here for advice on the subject. I think enough people have commented that if the topic you post means something to you sincerely....you probably shouldn't post it here for advice or share it at all.
My opinion? Send him a letter that is level-headed. Write your feelings out entirely; reread and share them with your family. If you feel you still need closure, send the letter. Aside from that, you can't fix whats happened but you can maybe volunteer someway to prevent this from happening. Maybe big brother programs or such?
When I think of anything done wrong to me as a kid, to me it's just overall more important that I try to make sure no kid goes through that same thing I did.