Hi, I'd like to help you with your question.
The effects of fathers in a child's life are numerous. Recent studies have shown that children who have fathers in the home do better in school, have a better sense of self worth and self esteem, and do better over all in life. When a child is abandoned over and over by their father, this can not only create a sense of instability, but it can take the child's feelings of safety and security away. They learn they cannot depend on those they love and that when someone cares for them, they will go away. Trusting others becomes a big issue.
It sounds like your son's father is taking out his problems on his son. If he is outright rejecting him after playing an active role in his life, he could possibly have a mental health problem.
If you have not already, try talking with your son about his father. Don't say anything bad about him. Just tell you son that Daddy has a lot of problems and that it is not your son's fault. Allow him to share his feelings about what is happening. Also, if he asks questions, answer them as honestly as you can but keep it appropriate for his age. Open communication helps tremendously.
It is also important that your son see a therapist. He needs a chance to work out his feelings, even at his age. The therapist could also guide you on how to work with your son so he understands what is going on, at his level, and help you if you decide to stop allowing contact between your son and his father. Talk to your son's doctor about a referral or search on line at http://therapists.psychologytoday.com/rms/.
If you do not already have sole custody of your son, you may want to consider filing for it. Talk with your attorney. You need to have control over how much exposure your son has to his father and vice versa. This is a hurtful relationship and you want to be in charge of how it affects your son.
It is always a balance to determine whether or not a a child should be prevented from seeing a parent. But given that your ex is putting his own needs ahead of your son and he is continually hurting him, it may be a good idea to prevent contact. Anytime a parent purposely hurts a child, it is usually better for that child to be removed from that parent. You may want to wait until you have your son see a therapist to start cutting contact. That way, the therapist can work with your son and you to ease the loss and help your son cope.
Here are some resources to help you:
Single Moms Raising Sons: Preparing Boys to Be Men When There's No Man Around by Dana S. Chisholm
The Single Mother's Guide to Raising Remarkable Boys by Gina Panettieri and Philip S. Hall
Longing for Dad: Father Loss and Its Impact by Beth M. Erickson
You can find these books on Amazon.com or your local library may have them for you.
I hope this helps you,