Dear Tam,
I agree with Elaine, a good church and youth group can do wonders for a child or adult's self-esteem.
I'm sure you let him know that he is loved and that you are proud of him. His grades are great!
Parental abandonment can have a lasting effect on a person, so for his sake, I would try to keep in touch with his father, let him know what's happening with your son and even if he's not going to come visit, (in fact when he's drinking he should not come around)--maybe that's why he stays away), but AT LEAST he could give his son a call once a week so he can have a connection with his father.
I know it is hard when we see our children hurting and my prayers go out to you both and his dad. I hope you will be able to get your son into some new ativities and that his dad will put his son in front of the drinking, step up and be father. Kids deserve both parents in good working order, because it's a tough old world to be growing up in.
May God bless and keep you all.