Tasha, if you can feel his ribs easily, he's probably not overweight. Cats do not have a set ideal weight that fits all cats, no more than people do. Some cats are naturally bigger, some smaller, and they are not the same weight or even close. As long as you can feel his ribs, his backbone and such, he's fine. Look at my cats- the older one (domestic) weighs about 9 lbs, and she's just right, as I can feel her ribs easily. My 10 month old (Maine Coon) weighs about 10 lbs, and she's definitely underweight, since all her bones are way too easily felt, and could be seen if she didn't have a long, thick coat.
But like hissy said, if your cat is drinking heaps of water, he should be checked out by the vet, as this can be a sign of illness. The vet would also be able to tell you whether your cat is overweight or not.