The brain is a muscle and will atrophy from non-use just like any other muscle. He needs to exercise his brian by reading, doing cross-words, engaging in conversation, doing things with his hands, perhaps a hobby, watching shows like Wheel of Fortune or The Price Is Right, etc.
Have you checked into the beginning stages of Alzheimer's or Sundowner's Syndrome? Older people get Sundowner's in the evening, it's a period of mental confusion when the sun goes down. It's a real medical condition too.