My ds is nearly 4 and just under 15kgs. He was a sick bub and had reflux, so had a slow start then he was a terrible teether and would go off his food and became dependent on his milk but once he finished teething and I got rid of the bottle he started to thrive but still battles gaining weight. We worried about him all the time for ages but he was always happy enough and not like he was sick all the time so in the end we thought as long as he was also eating fine which he does (he eats like a horse now) we have accepted that's just the way he is. Because he's so active with kindy and daycare we do give him an iron supplement though. Have you seen a dietitian? I would be weary giving him a formal as he may fill up on that and eat even less.