Here's how we did it...
I picked up 9 sheets of identically colored scrapbooking cardstock paper in a 12"x12" size. Very gently on the backside of the paper, I taped them together along the seams with removable tape. When flipped over, it created a 3'x3' large paper space to work with. Because I think mixed media is great, I had him work with crayons first. Because it's a large paper and he's a little guy, I kept rotating the paper so he could have equal access to all areas.
He used a variety of colors, all his choice, and drew "pictures" and created lines. As a side note, I really like the tin I'm storing his colors in. It's a metal Crayola tin with an attached flip lid that stays shut pretty well. I picked a couple of them up from Target. I cut pieces of paper sized to fit into the tin, so that way it's easy traveling and you've got everything you need.
Here is the last stages of the work. He's nearly done painting, and again, I kept rotating the work so he could reach all sides. He's standing in something called a Learning Tower, it's a safety ladder/platform that they can safely stand in to do stuff like this. I love the look on his face here, he's concentrating on what he's doing...
And here is the finished work. I already had 9 12"x12" gallery style canvases (they have a deeper profile), that had previously been in this space. I had taken them down awhile ago to change the images on them, and then never had a chance to put them back up. After he was done painting and it had dried completely, I carefully pulled the tape away from the backs of the paper, and mounted each square individually. I used good ol' Mod Podge Matte to adhere. MP on the canvas and then the painted work on top and pressed into the MP paying special attention to the edges to make sure it wasn't going to peel off.
The finished work looks great I think. Here's to hoping that he grows up to love art!