Does he take Ambien or a similar drug at night?

What other medications does he use? Statins for cholesterol?

Sometimes people can get stuck in a sleep stage, and be quite impaired. When they get up in the night alot to go to the bathroom etc, they tend to not wake up fully, because then it is difficult to fall back to sleep. So they learn this habit of only partial arousal.
This is when falls and accidents happen too.

If he remembers when fully awake, I don't think it is Alzheimer's.
But you might have him mention this to the doctor incase it is related to other medical issues he might have.