L.L.
Allow him to be creative with what he finds around the house...my daughter (who is also 5) is constantly making things out of what I would like to throw away. She loves to build things out of empty boxes, toilet paper tubes, etc...
...one really fun thing to do, if you can find a large flat sheet of cardboard, is to let him build a city on it. You can make houses out of empty boxes (oatmeal, Jiffy muffin, small boxes like that) and he can cut out the doors and windows, and it's a great chance to teach a child how to "wrap" a package (with construction paper, e.g.) and then they can decorate the houses how they want. Toilet paper tubes makes great silos, towers, tree trunks, etc, cotton balls can be used for tops of trees, bushes, etc, popsicle sticks can be used to make fences and playground equipment, as can string, toothpicks, etc...
The ideas for making a city like this are really endless...if you have a basement and a fridge size piece of cardboard (easy enough to find...ask around at stores, etc), then you can really let him "go to town!"
My daughter also likes stickers to incorporate into her drawings....
oh gee, there are just so many things you can do!