Mike, If your dog was skinny on a food with 20% fat, I can't see switching to one with 20% fewer calories. I assume you keep him wormed and their is no other condition that might make him thin and are feeding him an adequate amount.

A hard-running dog will have a difficult time maitaining weight with anything less than 20% fat. You might want to consider giving him some sort of extra calories. Maybe something as simple as some bacon fat drizzled over his food. Lamb and rice is good for dogs with digestive problems and I think Maxx Nut at WalMart has one that is 20% fat.

Maybe it will cool off enough soon for you to run some bushytails.