He sure dosn't look sickly or undernurished and that coat is GORGEOUS!!!
You say the vet gave him a clean bill of health? No worms or anything like that?
If a dog has poor health other things by this time would have started to waiver such as skin, coat, shedding, teeath, stuff like that. I also don't think he would necessarily have to be a miniature to be a small guy, just small for the breed just as peopel are! I was surprised by the insignifigant weight loss over those several months but he looks healthy, does he eat regurarly? Not so much a question of how much he eats but does he eat regularly on a shcedule and keep up with it?