I think if you want to do a room like that you need to get Milvicias input... over on the autism board.... its her thing.

I work in the arts with my patients as a therapist, getting them to be expressive and creative is my thang. The best way to get your little man to be creative is to steer clear of the kits, coloring books, fill in the lines etc.. Give him raw products, like pompoms, seashells, craft sticks, toothpicks, pinecones, paint etc. Not neccessarily all at once.. if he has developmental issues limit the variety, but let him pick what he wants to use.

Make a little work shop for him or a art tote out of an old laundry box. I made a cool one a few years ago.

Try not to spend money on it (or much) and try not to limit how messy he gets.