I know that it is more expensive than most other cat food brands, but I swear by Science Diet! When the cat is under a year old, you should ALWAYS be using food designed for kittens. When we adopted my kitten, Stanley, he was only 1.3lbs at 8 weeks because the family had been feeding him "whatever's on sale" instead of a proper diet. We add a little water to the dry pellets, and some times treat him to the wet food, and he has caught right up. He now weighs 7lbs!
If you can't afford Science Diet, I totally understand. But regardless of the brand, make sure you are feeding him food made especially for kittens. Some food for older cats has things in it that kittens aren't able to digest yet, so they wind up lacking nutrition because they just poop it out. Give it a shot!