3-week camping/hiking trip

My friend's son is going on an extended, school-sponsorded trip, 3 weeks, out west. He's not backpacking but camping and day hiking. The question he asked me was, "how many exchanges of clothing should he take, particularly things like underware?" I believe everything will be packed onto the school bus and he's packing a big duffle bag and a day pack.

If it were me....
Assuming no laundry???
Clean socks and skivvies for every day, +2 or 3 on the socks JIC.

If we is willing to wash his undies, he can get by with about three pair of undies and three pairs of socks.

But when I was his age, I could not be trusted to wash my clothes in that scenario!

One thing that will help him identify his duffle bag is putting something distintive on the outside like orange paint stripes or something. That will prevent someone else from taking it by mistake.

If it was me going on this trip, I'd take disposible underpants available here. Kids would probably get made fun of though.

When I go on motorcycle trips, I take my oldest underwear and t-shirts and throw them out along the way.

If he is under 14, then anything above 3 or 4 changes is overkill because he won't use them. But, if there are girls with them, and he is older, then 6 is probably good. He can wash once a week.

If it's an all-boy trip, maybe the leader can control it so they need less underwear. I used to call my troop together about halfway through the week at summer camp and tell them, "I've got good news and bad news. The good news is we're going to change underwear today. The bad news is that Chris will change with Todd, Bill will change with Jerry, George..." (Sorry, it's a tough day at work.)