The guy may be a lowlife, but unless he actually and materially aided these terrorists, there is virtually nothing the Feds can do. Prosecuting him for his speech and ideas, however vile, will give the government unacceptable power to silence anyone.
I believe you are correct on this. Unless he’s in some position of getting US citizenship shortly or such....all they can do is watch him. It’s like having a full-time brain-washer working his way through the US, and always finding dozens of people to work on.