I don't know if this would be OK or not but someone took a regular cement block like you would make a block wall with & painted it light bluish color to look like water & then on the sides, added fishes & some underwater type plants. You can see a pic if you go on Garden Junk Forum & Kirkus has just brought up the Inspiration Album that Purplemoon did.Albums are on the right side & click on "concrete things" I believe. The block could later be put in garden, fill holes with dirt & plant a couple of seeds of GS's choice & he can have some veggie's or flowers for his efforts. The blocks are less than 2 dollars, would need some craft paints & small brushes. Probably need a med. green, black, white, orange or red for bright colored fish & you can add little white to make red pinkish so everything isn't same color. An orange fish with black stripes going down his body would look good. A lighter orange would make a gold fish, etc.Should seal the block with spray sealer before & after painting even on inside of block so paint doesn't come off.If that is too hard you could use 1 of those wavy top straight cement pieces & use the rounded humps & turn it into a caterpillar! Have fun! Jan

Check with your county on this. I've seen exhibits made from Lego bricks at our state fair. Legos are what come to mind for me.

I think of wooden building blocks like kids play with. (or used to, now so much is plastic or legos like adella says)
Maybe he could make a grown up version of kids picture blocks. He could get or make some cubes of wood and either rout or burn (if you have a wood burning kit) a picture or design into each "side". So each side would be a puzzle for a different design. I've always wanted to make a decoupage set where you decoupage 6 pictures, but routing or woodburning might be more up your gs's alley. Or painting. The different sides could have different lay outs too, all long for example. He could choose themes that a teen boy would want in his room. Somethings from a favorite book or game for example. (My set that I had when I was a little girl had a train on one side that you made by stringing them all out which I mention just as an example that you don't have to stick to the rectangle.) They wouldn't have to be cubes either necessarily, but he would have to plan it out ahead of time.
Am I making any sense?
The one thing would be to make sure that each side is distinctive enough that you can identify by color or type of design.
I've attached a link to some kids picture blocks that have a partial picture one each surface that you put together as a puzzle as an example.
good luck
Kathy

Here is a link that might be useful: Kids picture blocks for example.