OP, I feel your pain. Mine takes a highly reactive medicine that keeps him from full sun, and he wanted to spend two weeks hiking in New Mexico with friends at 17. I said show me that you are ready in the next week. And of course, he still failed to wear the hat, the sunscreen,etc, so I said “Nope, you aren’t ready”. Like an anaphylatic reaction, we are talking life threatening here, so it’s hard to just hope for the best.
It is the hardest thing in the world, and I hope he learns to take care of himself properly. Keep working at it, with an eye towards him having ownership and responsibility. For the record, my son’s best friend is VERY allergic and when he forgot his epipen, I made him call his mom, who brought it over, and we pulled him from the group to give it to him. We did that two more times before he remembered on his own.