I would get him a letter written in the language of the country he's going to, which states he is diabetic, and what effects it may have, and what treatment will be needed.
Ideally, also get him a letter from his GP stating 'officially' that he is diabetic (you will probably have to pay for this.)
Ensure his travel insurance is fit for cover (eg All Clear, StaySure or other travel insurance companies that offer pre-existing medical conditions cover)
If he is going to a country where there may be issues over what he can take into the country, then check the country's foreign travel (or our own UK's guidance) as to whether he needs, say, a doctor's certificate to say he can bring in insulin, sharps/needles, etc, and they won't be confiscated at the border.
Hopefully this is all pretty standard, as there are loads of diabetic holidaymakers, and whatever country he goes to will have their own diabetics as well - sadly it's not rare.
I did all of the above in the case of my son, who has a potentially life-threatening nut-allergy, and in some countries (eg, Dubai IIRC) I needed a doctor's letter saying he had this condition, to give him permission to bring in his Epipen (sharp needle plus adrenaline) through their customs.
Also, and this, I know is 'Mother Hen' (!) tell him you DO WANT to get a brief text from him every 24 hour saying he's still OK. You dont' have to make a big deal out of it, but tell him if he loves you he'll do it !!!!! Better still, and, indeed, 'as well' identify one of his most reliable mates to 'watch out for him' as well, and give the mate your contact details etc.
Also, and I know this is probably 'Extreme MH' (!!), check your own passport is in order, and that you can book an instant Easy-jet flight out there just in 'case' (!) you get the text 'Mum, I'm fine, but I'm in hospital....' (or the mate texting you!)
If he 's going with a GF then brief her accordingly. (My DIL is now my 'in pro persona' MH!!!!!!!) (Yes, she carries an Epipen in her handbag - or so she tells me at any rate!)
All the above apart (!), I hope he has a really great time, and learns (safely!) how to combine having fun, having as near-like-everyone-else's youth as possible, and is sensible and safe for the duration, without too much anxiety (yours as well as his!)
All the best with it.