My first thought is, did you ask your DS why his medication was still in the pill box? What did he say?
I don't know what the principal means by "appropriate times," but I would wonder if your DS didn't want to take the pills on a regular basis in front of his friends/classmates; if, like my kid, he wanted to be "normal" and "like everybody else" for those 6 days, maybe?
It's hard to know what the group dynamics were, what the housing/rooming situation was, etc., but I guess I could understand that, at this age, the chaperones didn't feel as though it was appropriate for them to hover over your DS to ensure he took his medication. You trusted him with the pill box, rather than a chaperone, so perhaps they took that as an indication that you more or less trusted him to take his medication as appropriate, and they took a similar path?
I would talk with your DS about it and perhaps brainstorm some strategies for the next trip, either so that he doesn't forget (whether or not there's an adult present to remind him), or so that he feels like he can take whatever he needs to take without "making a spectacle" of himself. When my DS was a little older (9th grade) and out of town on a school robotics competition trip, we talked about how he would take his meds first thing in the morning, with a glass of water from the hotel room tap, before he even left the room for breakfast or whatever was on the schedule. That way, he had at least a little privacy and didn't have to haul the meds around with him, or run the risk of forgetting. Another idea might be to set up reminders on his phone so that he gets beeped or whatever to remind him, if he's the forgetful type.
Otherwise, for the next trip, you might request a one-on-one conversation with the head chaperone and ask, specifically, that they find a moment to pull your DS aside, outside the hearing and/or eyes of his peers, and ensure that he's on schedule with whatever he's supposed to be taking.