do you feed him just pouches or dry food as well? how old is it? i would give it 2/3 pouches or 1 tin a day and keep dry cat food available along with a bowl of water for the rest of the time. have you wormed him?
i'm with sazm - does he need worming? my two are 12 weeks old, eat two pouches each per day plus a good handful of dry food, plus half a bottle of cat milk between them (they love it!). But they do seem to eat loads, and have that leggy skinny tweenager look to them at the moment!