2 year old toddler son about to be 3

My toddler doesn’t grow or gain any weight since about 6 month. He weighs 20lbs. Took him a year to gain 5lbs last year. He eats a whole kids meal and eats some of his sisters food as mine when we go out to eat. He is also very delayed for his age. He’s seen every specialist and genetics ended up saying, he might have something todays science and technology can’t see. Has anyone had there kid like that. What can I do. I’ve tried many things.