Under American law, no.
I don't understand what he can be charged with, unless he had something to do with killing that CIA dude, or parttook in planning terror acts against the US, in which case he would be guilty of murder, not treason.
He went to fight for Taliban before they were at war with US, and it's unclear if he took arms against the US after the military action started.
We didn't declare war on Taliban, so technically he is not a traitor. I know it's a technicality, but that's what laws are.
I am pretty sure he won't be executed. But since there is so much hatred for this guy, he will probably serve time on some charge or other, and for his own good, because it would be hard to keep him safe in the US if he is freed.