Wa 'alaikum as salam DaReelG,
The first thing your friend needs to know is that what happened to him as a child was not his fault, not in the least bit. And it does not change who he is now (I mean, it does not need to affect his present life). Since he is Christian he may be familar with some of God's Mercy, and perhaps God's Justice as well. Remind him that his uncle will have God to answer to when he stands before Him on the Day of Judgement. As for your friend, let him know that he's "still your boy" no matter what. I would imagine he's feeling pretty vulnerable after opening up to you so he might need reassurance that you don't think any less of him now that you know what happened to him, and let him know how good it was that he did tell you, because that is a very difficult thing to do, to admit something like that.
He may have some anger over this matter, and if it's the case, that's okay. He should allow himself to let that anger out, but it is important for him to know not to take that anger out on others, because that will not help or change the situation. Leave the man who hurt him to be dealt with by God, because God is the best of Judges, and that man's sin will not be overlooked.
I ask God to reward any efforts you have made, and give you the strength to be the rock your friend needs to hold him up in his time of need, and I ask God to strengthen him as well and guide him towards Truth, towards Peace, and towards Success in this life and the Next. Ameen.
Take care brother, and remember to make du'a for your friend.
Wa salam.