my (almost) 2 year old won't gain weight.

Over the last year he has only gained 1.5lbs. He weighs 22lbs according to our scale at home tbt he loves to stand on bc he likes to watch the numbers. He has beencto the dr a few times this year for a few colds/allergies and I have told her he hasn't been gaining weight and she doesn't seem concerned. He does est but it's more like nibbles. I'll sit him down for a good meal and he eats tiny bites or eats the one thing on the plate he likes best and ignores the rest. I try to feed him myself but he wants to do it alone. His actual 2 year old appointment is on his birthday (12/12/16) and I'm worried they'll find something really wrong with him. I know that's me being paranoid but he's only just now starting to fit in 18 months clothes and it's bc he's finally gotten an inch or two taller. All in all he's a happy, healthy kid.. he's just small. Any ideas on why I can do to help him gain weight?