Buy an easel? http://www.easelsonline.com/Children...-p/ae1300.html

Wait until he doesn't need to do it anymore ("outgrows" it?)/hasn't done it in awhile and then paint.

Leave washable crayons around? http://www.crayola.com/canwehelp/staintips/index.cfm

Work on the computer and "teach" him how?

http://www.crayola.com/coloring_application/