hi there, maybe you should have your peditrician gave you some kind of vitamins to help open appetite or have him referred you to a GI , just to have him checked out. Imy daughter was the same and my GI recommended this great medical food call DUOCAL , which can be added in in kind of liquid he or she is consuming , I tried it and my daughter gain 10 lbs. you can only buy it online, no harm or side effect in it. it add calories. if he likes ice-cream for instance , you can add it . hope this help
My eldest was the same way. Everyone kept telling me she was just being a picky eater. Well come to find out her tonsils were huge and it hurt her to eat so she just wouldn't. My suggestion would be to get e referal to an ENT and have a swallow study done and look at his throat really good. She wound up having surgery at 5yrs old. She only weighed 30 lbs and 34in high. She made up for it after the surgery though gained like 15lbs and grew 6 inches in 1 year. Hope this helps.