You have nothing to worry about, the only thing that will make your dog lazy and fat is if he is not exercised.
If you were having second thoughts, I would strongly recommend you have it done, as aggression in male dogs (such as Pit Bulls) is largely due to the testosterone secretions of unneutered dogs. He will be less of a risk factor to strangers and be better tempered all around. Plus, if you don't want him copulating, it will largely be to his benefit to relieve him of that drive. Good luck!