Bruce looked skinny to me until he was about 3...he has always been very active and eaten whenever he wanted to. I know having food out all the time doesn't work for all dogs, and some people object to it, but it's worked well for him...he only likes to eat when things are quiet and will leave food for just about any reason so this way we were sure he was never hungry. A vet is the best one to tell you if it's just being a young boxer skinny or if he's underweight.