A good point.
Whether it's his fate to live or die after this, he should never be allowed to set foot in America again.
If he cannot be convicted of treason for a legitimate reason (the '2 witnesses' specified by the Constitution) then put him in a US Navy ship's brig for the rest of his life.
He would never escape, as he might be assisted in doing from an offshore prison, and he would never again even see the land of his birth.
When he dies his body can be disposed of like garbage, over the side, without ceremony...
Unless the ship sinks before then, in which case he can remain in his cell.