Nothing you can say or do I am afraid, I take it he is not an early to bed person, not that that makes much difference with dementia. It may be time to try some sleep medication.
I had terrible problems with my mum and when she was up I had to get up. Although I did leave a sandwich, biscuits and a drink in the kitchen for her and she went straight for this around 4.30 a.m, but she would never go back to bed and I would hear her constantly moving around the house. I put a lock on my bedroom door, it was too much when she would wander in and try to get me out of bed. Sometime around 7 I would get up and find her fast asleep on the sofa.
Some nights she would go to bed and sleep for a couple of hours then up she got, other nights she would be in bed only for a few minutes and there she was at the sitting room door, smiling at me. Of course I was not smiling, just so tired and desperate!
In the end I turned to the gp and she was prescribed sleeping pills, but not every night. I also used some over the counter products, but these did not work as well as the prescribed medication and the prescribed medication could not be used long term.