I'd have a chat with him - not in a 'I don't trust you' way, but in a way that shows you think he's able to control his own diabetes. You can then go over any concerns, but in a positive 'as you know, of course' way.
If you're especially concerned about the drinking, then maybe mention it, but in a way that implies it's your problem - you love him so you worry - rather than because you don't trust him.
I second the advice about telling his friends. Keep it simple and make sure they know hypo signs and what to do if your son has one, and when to get outside help. Also give them your number.
I presume your son's got or is getting travel insurance?
I believe Spain uses the European emergency number - that is 112.
I hope he has a good trip.