He can be compelled to show up at a proceeding where he'd have to give sworn testimony about the matter. He can refuse to testify on the basis of lawyer-client privilege, but he'd have to identify his client(s) in order to do this.
I'm thinking we're never going to hear his name again after next weekend.
If the authorities haven’t already attempted to question him, it is already over! As usual the good guys don’t want to fight. Trump may eventually win the battle(s). He is going to let the DS impeach him so he can have the Senate trial. Meanwhile the DS continues to walk all over him and by extension us.