Greece suspended the vaccine/testing requirement for visitors starting today, which included a booster shot. However, he should probably get it for his own better protection.

He can get an Uber-taxi with the app from the airport, it will be safer, as the taxi drivers will probably try to scam him for an extra 10-15 euro. He should try to use public transport or use Uber- taxis through the app for his stay, as some local taxi drivers will try to overcharge him when they realize he is not local.

He should get travel health insurance and a booster, as the Greek hospitals are not that good and easily overwhelmed by Covid.

These being said, odds are extremely high that he will be completely fine.