My father-in-law who has Dementia sleeps quite a bit of the day away. Unless you engage with him all day long, he will sleep. My husband tries to keep him awake by having him fold socks or towels, playing a card game, watching TV with him, looking thru photo albums, talking, but the minute you walk away or aren't talking to him, he is sleeping. Sometimes it's exhausting just trying to keep him awake! Then, as you could guess, he is awake at night ~ either getting up and down to go to the potty (for something to do), sitting out in the living room in the dark, wandering around downstairs, waking my mother-in-law up by talking to her, and won't go back to bed. It's like having a baby with their nights and days mixed up! Sometimes he gets confused and gets out of the bed and yells that he is wet and his bed is wet (and it won't be most of the time), or the bed is on fire and he can't lay down, and at times he will get belligerent about an issue when we go down there to try to get him back to bed (my MIL will ring the call button). How do we keep him up during the day other than what we are doing? How do we get him to sleep all night? We are on a tight schedule thru the week, up at 07:00, changing clothes/showering, breakfast at 7:30, etc, because I go to work, but it would be nice if he weren't begging to go to bed at 6:45pm!!! Telling him to stay awake when you are not right there with him is useless, he will sleep in the chair, or go in his bedroom and go to bed anyway. We are constantly getting up in the middle of the night trying to coax him back to bed, or trying to coax him into staying awake during the day, Help!
I know how very hard this can be. Don't let his doctors get off claiming this isn't serious and there is nothing they can do.
What your husband is doing with FIL is awesome. Keeping him engaged during the day is the best tactic. But in this case it doesn't seem to be sufficient.
My husband was quite agitated at night, and was up many times. That was my first concern, because I could not continue to keep him at home unless I could sleep. A zombie caring for a person with dementia is not a pretty sight. A sleep psychiatrist and a behavioral neurologist agreed on the drug we should try. It worked awesome!
But then my husband's excessive daytime sleepiness was an issue. In fact the doctors listened carefully to my detailed observations of his falls and then concluded he may be having "narcoleptic episodes" and literally falling asleep in the middle of a step! The prescribed something for that. It was our second miracle pill.
My husband still had dementia, but his quality of life soared on these medications.
So, he was taking something to stay asleep at night and also something to stay awake during the day. If you are opposed to drugs this sounds crazy, but it allowed our 10 year journey with dementia to proceed with some degree of normalcy in our daily life.
I am sure not saying that your FIL would benefit from the drugs my husband took. But I recommend tracking his activities and lack of activities for a while on paper and then discussing it with a dementia specialist.
(And drugs would not be a substitution for what his son is doing with him -- it would be a supplement.)