He might be lashing out because he's fearful of his health outlook.
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.