YAY ~ Good job from saving him.
The problem here is that he has been severely neglected, not neutered and not trained in any way.
I think the good news is that he is still young (3 yrs) and I do believe with the proper training you may be able to stop his aggression. Although it is going to take serious dedication and time.
Welcome to Dogwise.com - Dog Books is a great site for books on training and aggression.
#1 - Get him neutered ASAP
