Thin Labs are decidedly unnormal. How have you determined how much to feed him? Most Labs will eat more than is good for them and look for more. Have you talked to the vet about him? Unless he has worms or some other physical problem, you should have no trouble shoveling enough chow into him to reach his proper body condition. I once was given a high energy, 55 pound, 7 month old Lab that you could see her ribs. She didn't start putting on weight until I was giving her 7 cups a day of Pro Plan. A couple of months of that, and she was a sleek 70 pounds, and I cut back.
You have to remember, the typical Lab is over weight. Here is a link to a great illustrated guide to assessing your dog's body condition,
http://www.puppychow.com/products/po...condition.aspx Nowhere does it guarantee your Lab is typical. I actually had one Lab that refused to eat enough regular Purina to hide his ribs. I switched him over to a meat based, concentrated chow like the Pro Plan. He still only ate enough to barely hide his ribs, but was a fine healthy dog. Most of my Labs are on the small size and maintain ideal body condition on about 3-4 cups of Pro Plan at a year old. Another one I had, needed about 6 cups to hold the same weight. Here is a bunch of healthy Labs at their ideal body condition. They vary from about 6 months to 13 years.
The occasion was the one yellow Lab's 11'th birthday. The other yellow Lab is her mother.