Get him to talk. Tell the Chinese that he spilled all the beans. Then send him back to China so that they can convict him of treason.
Remember that suspicious death of a Chinese researcher in Pittsburgh a few months ago? Wonder what if any connection that has to this matter. Also it reminds me about the denied but not disproved story about Chinese researchers transferring viral materials from the Canadian lab to China several years ago. Weak and unconvincing denials have been issued a number of times, but they have the sound of a kid telling their parents, “I know the cookie jar is on the floor and my face is covered in chocolate, but it’s just a conspiracy theory that I ate the cookies.”
And some people say, yes, it’s just a conspiracy theory.