Very unlikely that a Treason charge could have been successfully prosecuted, even had they brought him to the United States right away.
Treason is, deliberately, a very difficult charge to convict on. We are not technically in a State of War with ISIS, so it is not enough that he merely adhered to ISIS and gave them aid and comfort. He would have had to commit an overt act of aggression against the United States that was observed by the mandatory two witnesses. Because the United States can’t even find a lesser charge to prosecute him with, it is pretty much a given that they don’t have the “overt act” and two witnesses necessary to charge him with treason.
There is a major reason that there have been no treason charges leveled since WWII and the immediate aftermath of WWII. The United States has not been in a state of declared war since World War II. It is very likely that the courts would only consider as an “enemy” within the meaning of Article III of the Constitution to be a country with which we were in a state of declared war. A non-state organization such as ISIS would likely not be considered an enemy within the meaning of Article III. Therefore, to commit treason, a person would need to commit an overt act of war against the United States, such as firing on U.S. troops or laying land mines. Merely adhering to an organization such as ISIS would not constitute aid and comfort to an enemy within the meaning of Article III.
The fact that they cannot make a lesser charge stick tells me that treason is out of the question.