Hey E.
My child is the same way. She's in the 25 percentile in weight and 75 percentile in height so she looks tiny and every stranger I meet can't believe she is 14 months old. Plus she is very active and only takes 40 minute naps each day.
She did spit up a lot up until 7 months of age and she is a very picky eater. She would never finish a formula bottle (even if the doctor said she should be drinking 2 ounces instead of 1 ounce - that sort of thing) she just couldn't finish it nor did I force her.
So you could try some pediasure, to make sure he's getting enough vitamins and nutrients. Find out what HE likes to eat, try new foods. He might like some buttery mashed potatoes or some greenbean casserole (like the kind you make at Thanksgiving) I give my daughter turkey/ham slices from that oscar meyer pack. She loves though, its gots lots of protein to keep her strong. Check out the Gerber Graduate Foods! There's all sorts of things!
Since he is walking, he's burning a lot of calories. Don't worry, but think about it, you don't want your child to be in the 100 percentile for weight, then he would be overweight! LOL So just let him do the eating!
but still see your doctor of course!
Good luck