I’m a not quite sure what exact info you want about Greece, but since am from Greece and live in Athens, I’ll try to give you a brief note.
Beware off
I) Taxi drivers in the Airport. If they see that you a first timer here (But just to be fair with them some of them and not all of them) they will (probably) double-charge you. So try to be cool and make sure that it is not your first time here, witch means that you already know about the taxi prices. But there is also a train that he can gets you to center with low price.
II) Pedestrian crossings. We do not always stop
III) Driving in Athens. Madness
Other than that there is no other reason to be anxious off, but prices are relative high at least for us who lives here. For instance a Frape in a good cafeteria has a range price from 3 to 5 Euros. Santorini is wonderful place and your nephew is going to love it. The temperature here now its about 30 to 35 Celsius, so it hot but the humidity is low. If he stays in Athens for a day or two he has to visit Plaka and eat Souvlaki, (this is a classic tourist attraction) and as for night he can go to the summer night clubs near the sea side, (near to Glifada, Elliniko suburbs etc) but he has to know that the face control is relatively strict.
If you need any additional info I will be very happy to give you.