To defend him not knowing it was a nazi salute or that it wasn't a nazi salute but something else is pretty silly.
He is Greek, not American. Every Greek (every European) kid knows what a nazi salute is.
He should therefore, unlike some Americans, understand what that salute stands for. He is aligning himself with an organization that, among other things think it is lawful for the unconditional slaughter (to the point of extinction) of people because of their race.
Everyone is entitled to their opinion and if you too think that's it is alright or cool to "support" such a regime then that's your choice.
The powers that be in this case thinks it is not.
Years ago many people believe that such things as the holocaust will happen again. It will happen, they say, when the generations to come forget what it was all about and (or) try to re-establish it as something of a cool retro.
It seems like this time is fast approaching, if not already here.