Upvote:4
is he still subject to the 90/180 day rule because he entered on an American passport
No. He's an EU citizen regardless of what document he used to cross the border.
or does it not apply because he is an EU citizen?
That is correct.
Is he legally required to get a Greek passport and enter on that?
Probably not. He can always enter on his Greek national ID. Or he can continue to use his US passport, and if the 90/180 rule ever becomes an issue, he can show his Greek ID at that point to show that it doesn't apply to him.