Instead of resigning, the President only needs to be consistent in applying the anti-graft and corruption laws to both friends and foes.
Strong words and warnings may work, but only when backed by political will and followed by concrete actions. There is no better way.
As for his offer to appear before a congressional hearing, that may not be necessary. In fact, the Senate is already proposing in a bill to give him Bayanihan-like powers to address red tape in government.