Is there a particular reason he wants a "paper pieced" quilt?
I'd suggest copying those coloring book pages onto fabric, get him some fabric coloring pens/crayons and let him create the blocks, you just sew them together. If he's into TMNT, he may be into coloring too LOL
Or as someone else suggested, applique them.
If he doesn't like those ideas have him draw the turtles using only straight lines, then you could make a paper piecing pattern out of it.
The reason I suggest this is because you could spend hours coming up with a pp pattern, putting it together and him not liking it because it looks weird. The TMNT are very detailed, and would be difficult to pp IMHO