Ryn, my DD2 is a skinny slip of a girl and at that age she was only 15kg for ages and ages but once she hit 5 she gained 4kg within a few weeks and now she has slowed right down again and is nearly 7 and would be 22kg tops. She is quite tall though which makes her look even skinnier. On average, they are 'supposed' to gain2kg a year, but if you know he's not gained wait for nearly a year but still eats like a horse, he either has a super fast metabolism and burns off most of what he eats or he could have absorbtion issues. Maybe try giving him full fat everything - buter, milk, cream and yoghurt etc and see if that helps?