These rules are supposedly valid until 1 May 2022.
User nk15 earlier mentioned the vaccine requirements have been dropped but I cannot find an official reference for that.
This site might be helpful for the OP
https://greecehealthfirst.gr/?lang=en
Originally Posted by
southlondonphil
Entry to Greece requires that the last vaccine shot (of a 1 or 2-shot dose) was less than 270 days or 9 months prior to arrival. Your son needs to get boosted otherwise he's likely not going to be on that flight to Greece
That's 'real world' practicalities, not politics.
https://travel.gov.gr/#/