My almost 3yr old is about 15ish kg. He's also about 98-100cm tall. My 4yr old is 16kg and 102cm. My DS1 is a fairly fussy eater too and some days eats and eats but most days he just picks at his food. While I do worry about it because he is so active, he is happy and sleeps well, wees and poos as normal, so it seems to the logical side of me that he's healthy. I don't think you DS's weight should be too much of a concern, but if you're worried about his health, then maybe take him to see a paediatrician if the GP is no help to you. The diet he was prescribed sounds very limiting, and if you're worried about his health I wouldn't be limiting his diet at all. Lean meats and protiens are a healthy and important part of a persons diet. Do you give him vitamins, maybe a child-friendly vitamin will help? Sometimes it help increase appetite if they have added vitamins.