I just remembered - Crayola makes a foamy clay product that has a light fluffy feel when you start working with it... its addictive to play with it... just pulling and kneeding it, etc. As it hardens over the course of a few hours, it gets harder and harder to kneed, less malliable. I think its inexpensive and will fit the bill! He can sit watching tv, reading, etc while his hands are pulling the modeling clay. I think its called Model Magic or something like that.

He can do a ton of stuff with it - its white out of the package. He could use crayola markers to COOR the clay! Its really cool stuff!