Safety

If he's spending his days in a chair look up saddle style wheelchair cushions and buy the best you can afford because sitting all day in the same position can lead to pressure ulcers. You may also want to explore getting him a tilt in place wheelchair with lateral supports and a headrest, there may be programs to help with the cost.

See if you can find a carseat cushion. They are slightly wedged. You put the wedge towards the back. The thicker part will be on tge front. See if this helps.

Yes. There are non slip materials that can cover a cushion that helps to prevent this slide.
It IS a problem, and especially if he is slender, and doesn't have a solid "seat to sit on".
Wishing you good luck, but this is and always has been a big problem. Discuss with his caregivers what solutions they have; they are the ones best adapted to this common situation.
Sure wish you luck.