A large piece of painters canvas, pre-primed. Draw roads, gas station, stores, fire dept., police dept., a farm on the outskirts with a barn, post office and any other town buildings. Trace over with a permanent marker. Now your son can color his own town and have it to use for his cars, firetrucks, small animals figures, etc.
We had one when I was younger that was done on a sheet with colored pencils. My brother didn't use it much as he was getting to old, but my younger sister and I used it a ton!
I always wished that I could have colored it the way I wanted because the barn was brown, but then again, I am an art teacher now...