"Out of complete disrespect for this committee, I assert my fifth amendment priviledge."
Say NOTHING else. NOT A DAMN'ED WORD.
But this guy is a Deep State critter, not a Trump loyalist. He doesn't like Trump.
So figuring that he's a lawyer and doesn't like Trump, my guess is that he will comply and he will testify.
But don't expect him to clam shut with the 5th Amendment.
Again, that's what we want him to do but it is not what he will do.