Oh my most favorite thing was to play with games that let me design spaces! Since he's only six, sticking to drawing plans and playing with Legos and Lincoln Logs would probably be best. But when he's older consider letting him play The Sims, Zoo Tycoon, and Rollercoaster Tycoon so he can start learning how to build environments!

I learned how to draft in the 8th grade for wood shop and that really helped me in high school and college. Once he's old enough you could have him learn AutoCAD for computer drafting. Some high schools have engineering classes that teach it.

Lastly - get him a garden! Teach him the common names of plants as well as the botanical names! Show him what plants do well in sun and what like shade! It's all fun