One year I found a small bucket of thin wooden shapes at the dollar store. There were circles, squares, triangles, etc. of different sizes. We glued them together and painted them to make mini ornaments for an 18" tree, and they turned out really cute!
The ones I can remember were a square with a rectangle and small rectangle put together to make a house, then painted like a gingerbread house. I overlapped the shapes. I even glued a tiny bit of cotton ball to the back of the chimney for smoke.
Three circles of different sizes made a snowman. A small square made his hat. I bit of yarn could be a scarf.
My husband and I were dating at the time, and we had roommates and friends over for this crafty project. I gave my husband a small tree for his apartment, and it was sports themed, so we painted lots of circles like different sports balls.
I remember there were little tear-drop shapes, too. I just painted those metallic gold and turned them so the point was down, and they looked like little glass ornaments or icicles something. There were also teeny star shapes. I used them as "filler" teeny ornaments to fill up the empty spaces on the tree.
When we had our shapes all painted, we turned them over and glued ornament hooks to the back for easy hanging.
I painted two larger star shapes gold and sandwiched an ornament hook between them with glue. Then I straightened the hook and wound the wire around the top of the tree to hold on the two-sided star tree-topper.
I also provided my husband's tree with a red velvet ribbon (I actually cut it in half length-wise so it was narrow enough) and some gold mardi-gras type beads to wind around the tree as garland. I found a mini tree skirt and short strand of lights (about 35 bulbs) at the dollar store, too.
It turned out really cute and it was fun to make. I'm sure you could adapt some things so your son can help paint the ornaments, too. I wonder if you could use foam shapes instead of wood? They might be easier.