His growth spurt could explain his thinness. My nephew is 10 years old and is extremely tall for his age. He would get a small pot belly a month or so before a growth spurt, we would think he is finally putting on weight and within a fortnight he would be a string bean again. He is just naturally tall and thin (exactly like his dad).

If you are concerned get him checked by a GP and maybe a dietician who may be able to recommend some specific foods which would help keep his nutrients up since he is a small eater. Some kids are small eaters and some are big. All depends on their metabolism I think!