Ohhhhh GROSS! I am so glad you are making him clean it! Cause and effect=natural consequence....great job!
I have to share this gross booger story with you all-it's NOT just kids!

I am a commercial interior designer, and part of our business is designing and reconfiguring cubicles in big office buildings. The cubicals are made from modular furniture-so all the pieces can come apart to make new desks, kind of like Legos

Well, you wouldn't BELIEVE what our team of moving guys finds on some of this furniture! There have been desktops that have boogers CAKED on the underside when they have taken them down, more boogers on the sides/bottom of desk chairs, nasty "mystery stains" on the seats of chairs, and piles of fingernail clippings inside desk drawers
YUCK!!!!!!!!!! I'm so glad I work mostly in the office!!