Well, My neutered Cat is Fat and Lazy compared to my previous non neutered ones. But at least he doesn't disappears for weeks at a time, returning always dirty and sometimes wounded, and eventually never return again.
Cat 1: Got husky, but he was still little when we did it. He was actually rather aggressive after the fact (Not really mean or anything, just... Did what he wanted when he wanted), and later he became an indoor/outdoor cat and was pretty lean.
Cat 2: Never got fat, but was never very active. Very docile and snuggly, but how much of that was from the neuter and how much if that was from being manhandled since he was a kitten, I can't say.
Cat 3: Kind of a mix between the other two. Big, dumb, friendly orange bastard who likes to make noise and break things. He's large, but not really fat. He runs around the house a lot.
They may, may not. Don't over-feed him and do make sure to play with the cat. If you set out a huge amount of food you're tempting him to overeat. It would also help if you had another pet around to keep him busy.
There are few things more cruel to an animal than castrating it. It completely changes the behavior of the animal and turns it into a soulless creature just waiting to die.
Don't do it. If you absolutely have to do it it's because you can't have a pet.
If you can keep him inside and deal with the consequences, bravo. If an outside cat, you should definitely 100% neuter. The world doesn't need any more stray cats getting run over, dying of disease, etc.