Yep, my son is really skinny too. The adjustable waist pants from Gap are a
no-go, I can't tighten them enough! Levis are the only jeans I can find that
work after trying jeans from Gap, Old Navy, Gymboree, Arizona's, and Walmart
brands (rustler and wrangler) None would work and I wound up with a closet
full of jeans with tags still on them.

Also the elastic waisted pants/jeans from Lands End or LLbean work well and
that is what Im stocking up on for school wear.

He mostly lives in jogging/sweat pants though.

Good luck!

Kari