I second everything that angela mentioned and personally, I think that the stuff they put in pediasure is not good for your child. I also don't understand the concept of fattening up a child just b/c they are small when they have no other medical issues that compromise their nutrition status. Also, those, "supplement" bars they make are pure junk. I was given some of those for my son by a relative who is a ped, it was so sickening sweet, I felt it was inedible. Both my DH and I were very disgusted that a product made as a, "nutrition supplement" was basically just a big block of sugar.
If you think your kid is small, well, I have one who is smaller. He is 4.5 yrs old and 27 lbs. He eats a very healthy diet, is developmentally on track and a happy kid. I got a LOT of pressure ever since he was a baby to supplement or switch to formula, b/c family members thought something must be, "wrong" with my milk, that I had such a small child. When it was time to start solids, everyone was giddy with excitement that he would fatten up. Nope. To everyone's disappointment, he was still small. Genetically speaking both DH's and my side of the family have small, scrawny kids, so really I don't understand why both sides of the family were so concerned with how small my children are. I'm at the point where I am not worried. However, I can empathize with the constant worry. I worry a lot, but in the end I keep reminding myself that SOMEONE has to make up the bottom part of the growth chart, and that someone is my 4.5 yr old. The good thing about having a small kid is that I am more liberal with treats like ice cream. Also, is your son very active? That could be why he is skinnier too. I see kids who are overwt and I guess I feel that I'd rather deal with having a child who is on the scrawny side than a child who is overwt and sedentary. BTW, in the long run it isn't really going to matter. My brothers and I were literally the smallest, runtiest, kids at school all the way up until high school, now we are avg ht and normal to a little bit overwt. You would never have guessed that we were scrawny kids.