B.A.
Hi M.,
You should try feeding him a lot of carbohydrate foods like pastas and breads. Hopefully, he's like my daughter and LOVES mac n' cheese, spaghetti, PB&J, fettuccini and top ramen.
My daughter was underweight while an infant and becoming a toddler because she had GERD. She would vomit after every single feeding and still does vomit sometimes if she gets too upset or just out of nowhere.
We switched her from regular milk to soy and she eats nothing but pastas, eggs, cheeses and things like that. She eats veggies also, but she's not too keen on them and I won't force her to eat something.
Hopefully some of that helps!