Get him neutered. Part of the reason some people might say that dogs get fat after being fixed because some are much more calm. But with proper exercise and the right diet that should not be a problem. If he is that aggresive with other male dogs you could have a dangerous situation on your hands at some point. And don't worry he will still like to play & be sweet like he was before. For being sleepy (what steno. said) that is just after surgery and lasts for a few hours or sometimes overnight. Most dogs seem to be almost more happy than depressed after being fixed.