I think we need a lawyer to weigh in on this particular point.
I’m a lawyer. The House could subpoena him. He’d be required to show up. If he doesn’t want to testify he can plead the fifth. The reasons for pleading the 5th are between him and his lawyer. The only way they could force him to testify is if they offered him blanket immunity. If they gave him immunity, and he still refused to testify, they could jail him for contempt.