What to do if I have been raising my son since birth with no support or visits from his father, who now wants to become involved?

If you don't have any orders in place which have officially declared him "dad" and he is not on the birth certificate, then he is not a presumed father-- which means you do not have to let him see the child or the child go on visits with him.  You can make him go file the paperwork to be declared "dad".  If he does file for access to your son, you can file a counter claim to terminate his rights or alternatively to limit his visitations.

If there are custody orders in place, that grant him access to the child, then you need to get them modified to restrict his access if he is insisting on visitations.

If you don't have orders in place, you're in good shape for the time being.  You may, however, want to be proactive at getting him officially out of your life.  Considering his complete lack of support of the child for three years and his abusive history, you may be able to terminate his parental rights to the child.  You certainly have grounds-- but the eventual decision will be up to the judge.  Before you file a petition to terminate, visit with a couple of family law attorneys in your jurisdiction.  (Just arrange for consultations initially.)  By talking to a couple of them, you can get a feel for how your petition will be treated by the local judges.  They can't guarantee you a result, but they can give you a historical summary of the success of cases similar to yours. 

Your final issue is what happens if you pass away.  It's going to be very hard for your parents to prove the abuse issues after you are gone-- so the more you can get done now-- the better.  If you're just not ready for that fight right now, then make sure that you visit with a probate attorney to make sure that your paperwork is in good working order to grant one of your parents custody of your son once you pass.

Based upon your well founded fears stated in your question I suggest that you consult with a family law attorney as to what your legal recourse is with respect to child support, custody and visitation issues and based upon what has been discussed make an informed decision as to how you wish to proceed in dealing with the situation with your son's father.

Potentially a petition to be filed in the county courthouse addressing the issues addressed above or for an injunction against him. You should also establish your desires in a custody request for your child in the event you pass while he or she is a minor.