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Different things may be causing this. Some speculations- depression which is not managed (he may need to discuss with his MD different medications or dosage if his mood is still low. Irritability and anger are common in those suffering from depression.
Individuals who are retired and no longer find a job or a title to identify with and occupy their time also can become depressed.
Thyroid dysfunction as well as lower levels of testosterone can cause changes in the mood.
He may benefit from doing something that makes him feel productive instead of stagnant. Talking with a counselor may also help. There are different options to explore depending on what he is willing to do. If he agrees with your observation and wishes to get better, there are things that can help.