I am down to 2 pairs. I don't know where they are going but they aren't in the laundry, or in the bathroom, or in his room... or anywhere I can think of. God i hope he isn't throwing them away!
We had a similar issue with his socks. he would go to school with them and then come home without. It took forever and some stealth work but we figured out he took them off at lunch and threw them away. He obviously cannot do that with his pants.
So where are they going?