Added to this list, good idea to scan his passport and bank card/account details and have him send it to himself at his hotmail account and to yours. He can then access this any time he needs to, and so can you if you need to do it on his behalf.
Make sure his passport actually has your name and number in the right place for emergencies
One important one if its Greece, drink plenty of water every day. Dehydration is otherwise likely and very dangerous (he will also suffer from alcohol more without it) This also helps guard against sunstroke.
He should know that it needs LOTS of suntan lotion before sun to work property (and will ignore this anyway!)
Phone.
Turn off DATA roaming completely. Just checking a few websites could cost him £100s. He has to go to cafes to use their wireless instead.
Make sure phone roaming is activated before he goes with phone company
Code in a ICE (in case of emergency) number into his phone ie yours
Agree its a great idea for him to buy a local SIM card there which the shop will help him top up. Depends on whether his friends are doing the same.
Pack condoms, Greek ones look different (the condom packets that is
)
Do not hire mopeds/scooters (he should need bike or driving licence for this but some dodgy ones will rent anyway). This is a huge cause of injury in Greece amongst young people generally, not just amongst tourists
Avoid jet skis
Make sure he has the new EC medical card and its up to date. Also private travel insurance.
I'm sure he'll be fine :)