A friend of our family's is a pediatrician, and he gets SO angry at pediasure comercials. . .for a couple of reasons (and this is all pretty much a summary of his frustrated rants.
First, there are a lot of overweight kids in the US (but that doesn't apply to you at all), so advertising what is essentially a calorie fest is SO irresponsible.
Second, because he says our bodies don't register fullness as well with liquids, so the kids don't really learn to listen to their bodies for hunger signs.
Third, "average" is just that--an average. There will always, and should always be people above and below that average. It's just normal for people to grow and change at different weights. My brother was underweight, and I was above average (despite being a ballet dancer, go fig). When we reached adulthood, we were both healthy weights, it just took him a bit longer to reach it. Kids aren't supposed to all be the same weight. . .they're just not.
Fourth, pediasure sets kids up for terrible habits as far as nutrition. . .drinking your calories is just a bad idea, but pediasure says it's ok (and implies through it's quasiscientific name that it's medically sound.
Finally, he says (over and over) if they're not actually sick with cancer or something, kids don't need pediasure. They thrive on healthy diets--lots of fruits and veggies and clean protiens and dairy--just more of it that people actually dieting should have. Their bodies work best with good quality food in them, not processed stuff.
He's been a doctor forever, and remembers how angry he was when he first saw a product designed for sick kids being marketed for healthy ones. It makes no sense---like giving healthy kids insulin injections because it works for diabetic kids. Think about it, Pediasure has been around for a LONG time, but only recently has it been advertised for healthy children. If it was so good for them, it would have been advertised long ago. They're just trying to increase their market share.
Just some thoughts, and probably ones you've already looked at. Seriously, though. . .if your kids are healthy, just underweight, why rock the boat with an overprocessed product? Why not just feed them good food.
And, my last little thing: I teach 7th and 8th grade. I have the students for 2 years. There are tiny kids in my classroom and kids far larger than I am. I watch these kids grow and change through puberty, and many of them go from tiny fragile to hugely tall basketball player size--in 2 years. Your son is just at the beginning of this process, and you don't know how much his body will change during it. Kids aren't an "average" size. . .they're their own personal body size. Maybe your kids are just on the small end of that scale--and that's cool. Plus, I'm sure there's a soccer team out there that would kill for your son---he's the perfect forward, small and energetic---not to mention rock climbing, caving, track, cross country, etc.
anyway, good luck with all of this. . .and with your almost teenager!
--paisley