Tips on stopping Cat obesity?

Ok i have a cat who's barely a year old( basically still a kitten he's not a full grown cat yet) and he's already marinating in his own fat. He sleeps, eats and poops/pee's and that's his basic schedule. Even at night. How do I get my cat to loose weight? I was thinking about not feeding him alot but I have 1 other cat and he gets into her food. And I go to school and my parent's work so nobody can monitor. Any tips? also i have a girl cat(kitten) who is nervous in nature, and she's afraid of the cat I just spoke of, Coon. I keep them separated but Coon is just kinda mean at times and it makes her skittish. But they play together too, but Coon is a month or a maybe a few weeks older and a great deal bigger. How can I get things to work out between the 2? I don't want to give him away cause he's a sweet cat they both are. Please help.