Please get him neutered. He will be healthier and a lot more managable. There is a possibility that neutering will not help with his dog aggression but most likely it will. There should be no problem with weight gain after the surgery. That is basically a myth. He won't all of a sudden lose all of his energy and want to sleep all day. With proper diet and exercise, he will be just fine. I have never seen a dog suffer depression from being neutered. Some are pretty sleepy and a little sore for the next few days but thats the worst I've seen. The biggest problem after a lot of surgeries is trying to stop the dog from being active when they are supposed to be resting!