His crimes don’t meet the constitutional definition of treason. And it may be difficult to prosecute him under US law as (1) he may have done nothing illegal while *in* the US so there is a jurisdictional issue here, (2) never has an intermediary been prosecuted for espionage, only the original source. The best bet for US prosecution is that if he encouraged others to turn over classified information to him he could be prosecuted on conspiracy charges.
But in the mean time he is facing sexual assault charges in Sweden. It may be a while before the US gets a crack at him.
I agree that what he has done has terrible consequences. He is truly an enemy of the free world.