Louis is a bit like that at the moment too. He's 13 months, weighs about 12.5 kilos and sometimes looks skinny, whereas 6 year old William, same height, weighs about 13.8 kilos and looks absolutely right.
Both have Burns in the morning and raw food (Natural Instinct) in the evening, plus a variety of training treats. Louis gets about 30gms more Burns than William and about 30gms more Natural Instinct too. I've just increased it to that in the last week and can see a difference.
I'm sure it's just an age thing as Louis is fit and healthy and expends a lot of energy. At this sort of age they've reached full height but still need to body up. Some do that quicker than others but I don't expect Louis to look fully mature for a while yet (and as far as his brain goes, who knows, could be a very long wait)