Your son is very emotionally mature to be able to discuss this with you...at the age of seven! Most children that age have difficulty grasping what happened, and although it is saddening to you to hear him, please take comfort in the fact that he is not only able to process it (finally without violence or hate) but he has trusted you enough to talk with you about it.
Please find a good counselor or therapist. It sounds like he's ready to deal with and work through this issue in a healthy and positive manner. Judging by your post, it seems as if you have done a great job of discussing this with your son (not laying blame, listening to him and showing him love), but it might be worth its weight in gold to seek the advice of a third party.
A good counselor or therapist will have many techniques to assist both you and your son as he grows up. It may very well be important to have an impartial party who isn't mom or dad listen to him and help him. The old days where only "crazy people" sought therapy are gone; there should be no stigma attached to seeking help with a major life issue.
You've done the right thing by listening to your son and not just trying to cover it up, forget it, or lay the blame on dad. I wish you the best of luck with this!