Have you asked the school study abroad office?
I’m sure these are questions they’ve heard before.
Is the school he’s attending a US Branch or English school? I’d imagine either would also have local staff that has dealt with these issues with US students.
I’d start there, if you haven’t - and yes, I wouldn’t send pills yourself through the mail unless you know it’s legal. I’m wondering if he may need a medical diagnosis/appointment local…but I have to think these schools have had these questions before.
Good luck.