OP, sorry to hear of your experience. I know every case is different, but I just wanted to tell you about a wonderful man I know, who was adopted because he was conceived through rape. Although he knows the circumstances of his adoption, he quite rightly talks of it without taking any of the shame. Why should he, an innocent, carry the burden of what someone else did? He's one of the nicest people you could ever know. He's a whole different person to his dad, and it's the same for your own son.
I can see how it could be hard to see the physical likeness in your son, but it doesn't mean he's the same person by any stretch of the imagination. He's half you, too, remember! Plus, like someone else said, you are bringing him up to be a wonderful, kind, human being.
I am sure that seeking help for yourself to get through this, will make all the difference to how you live this difficult situation out. It can't have been nice for my friend to find out he is (in his own words) the product of rape, but his pure, sweet soul, and his courageous and honourable outlook on it all really shines. He's a real salt-of-the-earth person, and others are all the better for knowing him.
This situation you experienced is history, which isn't meant to devaluate how awful it was for you, but it is meant to show how the future can be much brighter, and doesn't need to be tainted by this dark shadows over you and your dear son. Best of luck with your journey towards that freedom. I hope the story of my friend can give you hope that it's possible 