How often should I wash my son's underwear?

He's 4, he finally grew out of stubbornly going full commando, and he got a habit of grabbing new pair of panties from the drawer every time, which I support. Problem is, he takes them off completely whenever he goes to the bathroom, and after that he refuses to take them back on, and grabs a new pair. Which is 4-6 pairs a day that he only wore for a couple hours.

We have 10 pairs, and we often end up with having none, what I do?

Yes, I talked to him, no, negotiations haven't been successful 🥲