After posting below, I had this thought. The DOJ should charge him with perjury and make him assert that he accepts the pardon. Then he can be compelled to testify because he has immunity. Apparently the. absence of immunity can compel him to testify or suffer contempt of court.
If he chooses to reject the pardon, then he can assert his fifth amendment right to not incriminate himself. But that opens him up to prosecution for the crimes he was given the pardon for.
I think it’s undisputed that if Fauci rejects the pardon, he can claim the fifth amendment and not testify against himself. But does it also hold that if he claims the fifth, he automatically is implying that he rejects the pardon, opening himself up to criminal prosecution?