Go out of your way to get him involved in a sport with your son. It will give him character, burn off excess energy, and improve him physically and mentally.
It sounds like the mother isn't raising the kid properly. That's not something you can solve, and it's going to be a chip on that kid's shoulders the rest of his life. Most kids in his situation don't know how to deal with that, end up looking for stability outside the home, end up making the wrong kind of friends, and end up in jail or on drugs, etc.
Get him in sports. Give him an outlet to blow off steam. Or keep your son away from him and wash your hands as another boy from a single parent house falls through the cracks and gets ground up by the system.