Yes, please listen to the person who warned that Pediasure is not a supplement to infant formula! Please don't do that unless your doctor specifically tells you to.
Do ask the doctor what else to do, maybe extra bottles instead of extra-strong formula. But keep cool: I don't want you to go through what our friends did: Their son was tiny. He ate and ate and ate and by age two was still very small but developmentally fine. Doctors sent the family to pediatric gastroenterololgists to see if he was digesting properly, to this specialist and that specialist and in the end some doctor referred them to an eating disorders program at a university -- which they found was aimed at teens with disorders, NOT at toddlers at all. That's when I think they realized this was out of hand, and doctors were not at all sure what to do. Then one doctor finally looked at the parents and said, basically, your kid is small because you're both small, and he's fine. Both parents are, frankly, short; and the dad is both short and very, very slim; he eats like a horse all the time and remains very slender and small. Nothing wrong, it's just his build and his metabolism.The kid is fine but just is small like his dad and always will be --- genetics will always win! But early on, doctors didn't know what to do with a baby and toddler who wasn't fitting the stereotype of an infant who's plump and getting plumper. In other words -- listen to doctors, absolutely, and if your son's developmental milestones aren't coming along as they should, definitely have the right tests done and soon. But be aware that not all children are little butterballs, and once they are very active, they get very slim sometimes, especially if predisposed to being smaller kids. Take a look, too, at the genetics -- are you, or is dad, or is a grandparent, slim and/or short and slim? That could be a clue. So is the fact he was a little premature -- has your doctor talked to you about how long it takes even slightly premature kids to get onto the "percentile" curves?
So don't dismiss any real worries, but if he is otherwise healthy, do not be unduly worried either.