My dad is 74 and has Parkinson/Dementia. I have noticed that he acts child like a lot. He admits one day that he has really bad short term memory but then the next day he says he is completely healthy. He acts like he is a teenager or younger child at times.
Lewy Body Dementia is often called the "roller coaster disease." I swear that my husband went through a period when I called it the pogo stick disease -- he was constantly changing from "up" to "down," with no transition in between.
Especially if you are his primary caregiver, I think that joining a support group for caregivers of persons with PDD or Lewy Body would be very helpful for you. (It sure was for me!)