What country are you in?
My DS sounds like he is built a lot like yours. I haven't had him weighed in a few months, he is due to go in two weeks, but he is about 33" tall and probably around 22lbs. He is a good eater as well, eats a good variety and always enjoys his food. My husband and I are not large either, I am only 5' and am usually around 110lbs (when I'm not pregnant!
), and DH is about 6' but is quite slim. Neither of us have ever been big, and I think DS takes after that.
It doesn't worry me that he is in the 5th percentile.
I do make my own smoothies though with fruit, greens, yogurt, a bit of juice, and I add omegas and vitamins for him. Probably a lot better to do it that way than buy something full of synthetic stuff!