What do the 3 meals and the 2 snacks consist of..is he exercising playing sports , what does he do when he is home does his Dad get out with him...How about the food you buy in the house for all of you, would you say you stick to healthy foods, veg, fruits some protein ? Buy no foods in the house that he cannot eat, so have a range of heathy snacks , the stores are full of foods to buy..would you say the rest of the family are the right weight and eat healthily ?
I would suggest changing his diet. Have lots of lean protein and even have slightly under what his calorie intake would be normally. Mostly just saying that he can eat when he is hungry, but not over eat, even if it technically says he can have more according to his height and weight. Try that for a while, and if the problems continue, then go for the second opinion. good luck.