Your son has been hurt by your ex. It is completely understandable that he doesn't want to interact with someone who has caused so much pain.
Relationships don't come equipped with a magic reset button. Just because your ex has made some changes in his life doesn't mean he gets a pass from your son for the last 7 years. He has lost your son's trust. If he wants it back, he has to earn it. The problem comes when your son won't interact with your ex to give him a chance to regain that trust.
I believe in trying to forgive others, especially when people are trying to make amends. That being said, forgiveness isn't always easy, especially for deep hurt over a long time. As such, your son may not be ready to forgive your ex just yet. So don't force him to talk to your ex. He's not likely to see those interactions in a positive light if he feels forced.
You do have a choice here, though. You know your ex. You know if these changes seem genuine and if they are likely to last. You can evaluate if a relationship between your son and your ex is likely to be a good thing or just another setup for pain and disappointment. If you feel that your ex has made real changes and that such a relationship would be worthwhile, you can work to help your son begin to forgive and give his father another chance. Talk to your son about the changes you've seen in your ex. Let your ex have some indirect contact (birthday cards, letters, etc.). Help him to see that your ex is becoming a better person and can be a positive person in his life.
If you are feeling brave, you may want to talk to your son about the pain he feels because of his father. Talk about how he feels, what he's most upset about. It's not going to be a happy conversation. But letting all that out with you (someone he feels safe with, in a safe setting) will help him to heal. Keeping it bottled up inside won't allow forgiveness to happen. And even if he never feels ready to forgive his father, talking about it will help him come to terms with it. He will learn how to accept it and overcome it.
Finally, both you and your ex need to realize that this isn't going to happen overnight. It will probably take some time for your son to warm up to the idea and to forgive and heal. Just be patient and encouraging, but don't force it. I wish all three of you the best of luck.