A picture of him standing from the side and top would really help. But the one of him sitting to me looks like what BowWowMeow says, needs more muscle mass. They do fill out and yes a bit thin is better than a bit heavy. The way I learned to tell if the dog is to thin is to take your hand, make it flat and rub your finger across your knuckles, then run your fingers across the dogs rib cage, they should feel the same for ideal body condition. If you make a fist and run your finger over your knuckles and the dogs ribcage, if that feels the same, dog's to thin, flip your hand over, palm up, and run your finger over the back of your knuckles, over the dog's ribcage, if they feel the same the dog is to heavy. Gosh, I hope that made sense. It is hard to describe that method in writting lol.