Little person eats, bit is still skinny

Natty

My 2 year old son is tall and skinny. No matter how much he eats and how much weight he gains he never gets any more body fat. He had baby fat as a baby, but then he started shooting up like a weed. It makes me feel bad because when he moves about I can see his ribs. He eats 3 meals and snacks all day. Does anyone else have this problem? What did you do about it?