FS beginning to show issues regarding food

P, my oldest had major food issues. I found out (after he'd been with me 2.5 years), that he went hungry with bio mom. He ate food in his roo; he would take it as soon as I bought it home. I now lock up cereal and anything for dessert. He'll eat fruits also. But he's now down to where he eats "normal" portions (as normal as can be for a 15 yo).