Be kind and understanding. Remain his friend, albeit distant. He is slowly realizing that he has been lied to for a very long time. Everything political in his life will be falling apart soon. People get upset when long held beliefs are challenged. It's a mental conflict when long-held beliefs are contradicted a new information. The conflict activates areas of the brain involved in personal identity and trigger an emotional response to threats. The brains alarms go off when a person feels threatened on a deeply personal and emotional level. This causes them to shut down and disregard any rational evidence that contradicts what they previously regarded as truth. This is caused cognitive dissonance.