How do I keep my father-in-law from slumping on the couch? We've tried to prop him up, but he always manages to slump down. He has dementia.

Try a foam wedge with the thick part towards the front of the seat. Also try a slight angle backwards and try an easy chair with arm rests. A narrower seat will also keep him from falling sideways.

I use a swiveling upholstered chair that has side arms and is fairly straight up and down in the back. That helps support her posture and the arms help with the side slump. Sometimes I use a small pillow on the side to keep her from slumping sideways. The swivel chair is really helpful for positioning her with the Hoyer lift I have to use because I can approach the chair from the easiest side and then turn the chair towards the TV. She seems to like to spin in it! I can also use the Hoyer sling under her as a draw sheet to reposition her.

Finny2: Since your FIL has dementia, he has lost core strength. Try a wedge.

How about a posture corrector? I cannot copy and paste the picture but it wraps around the waist then brings back the shoulders:
try this website:
https://www.goodrx.com/well-being/movement-exercise/do-posture-correctors-work

Perhaps get him a recliner chair and angle it so he can't slump forward.
If no recliner, try re-arranging support of pillows.

Gena / Touch Matters

As already suggested, a recliner or even a geri chair.

Mom sat on an electric recliner to minimize the slumping from loss of core strength due to dementia and being wheelchair bound. When she sat in her wheelchair it was the absolute worst. There's no way to "outsmart" this situation. By the time mom was seriously slumping over, she was at the end of her life and passed within 6 months. For Dad, he passed with 3 months.

Best of luck to you.

Get him in a CHAIR! Then the slump would be on the...floor.
It is IMPERATIVE that caretakers MUST be cleverer and smarter than the dementia patients!

Regular PT can help by working on trunk strength to at least try and keep it from getting worse. My husband's R side is naturally worse due to Parkinson's. I have him sit all the way at the end so if he leans over the arm of the couch partially supports him. Sometimes a pillow is needed behind and beside him.

My mother also has dementia.

After hip surgery 4 years ago to repair a fracture, my mother's (96) head began to tilt to the left. At the time her PT would gently stretch her neck to keep the muscles from tightening to where she couldn't hold her head up straight.

Mini strokes were suggested as the cause, but without an MRI, there's no way to be certain.

Now, 4 years later, she sits in a wingback chair and I prop a pillow on her left side to at least keep her head somewhat upright.

It doesn't seem to bother her at all, and I don't know of anything that can be done at this point to correct the leaning. She has also lost a lot of core strength and bends at the waist when walking.

My LO was slumping in the wheelchair. Losing trunk strength. A recliner was suggested by the OT.

Getting in & out of a recliner becomes the next challenge.

Hi Finny! From what I have read, the leaning is common with dementia. I don't know why specifically, but I have seen it happen with my MIL. She lost core strength and was unable to hold herself upright in a seated position. Here is a link to a question on this site about this as well. I hope you find something that works to prop your FIL up.
https://www.agingcare.com/questions/people-with-dementia-start-falling-when-sitting-198325.htm?orderby=recent

As a person with dementia declines they loose Trunk Support.
A Wheelchair with a back that reclines might help.
Rather than pillows to prop try using wedges they can be adjusted easier and are firmer than a pillow.
The couch might not be the best to begin with a recliner would be better if you do not want to go with the wheelchair. A recliner would offer more support and if you get a lift recliner it will be easier for him to get in and out of as he declines more. (The problem will be that he will forget how to operate it)