Depression is common after bypass surgery. As ScalpMassager says, he may be afraid, or he may be depressed. He might be a candidate for medication to help his mood. If the cardiologist won't prescribe anything, the primary care doc ought to be willing to. I would describe the problem to the doctors in terms of mood, and that might ring a bell for them. Good luck.
He might be lashing out because he's fearful of his health outlook.