Have you heard of Couchsurfing and Hospitality Club? There are a lot of nice people there in all kinds of cities who would be very happy to help your son by receiving a package. That's easier than trying to find the right post office and figuring out where your package is in a certain country. Also, in some countries they don't have post restante so the package will just be returned. I am not sure what the situation is with Greece and Turkey. There are quite a few hostels in Istanbul for your son to stay in. He can get some visas in Azerbajan as well.