My dad keeps sliding off his chair at the kitchen table because of his involuntary movement.

There are gel type pieces of "material" that can be placed on a chair that make it much more difficult for a person to slide. (remember a few years ago the product that was advertised to place on your car dash that would prevent stuff from sliding off.. it is sorta like that, or the old "wall walker" toys.)

A tilt back wheelchair would also be an option. My Husband had a high back, tilt back wheelchair ordered by Hospice. The high back made it more comfortable so his head was also supported. (if he currently is not using a wheelchair this might be met with some resistance)

A combination of the two would make it almost impossible for him to slide out.

I second lealonnie's suggestion, a tilt in place wheelchair with lateral supports, a head rest and a ROHO seat cushion became the place my mother spent all her time (when she wasn't in bed). It was a big investment but there were government benefits that paid a significant portion, and it was worth every cent.

Your father needs a wheelchair that tilts backwards.....to prevent this very thing. He needs an evaluation by a PT or OT to be fitted with the proper equipment.

Then dont seat him at the kitchen table. Get a bedside table or something and let him sit in the chair in the living room, or other chair that he won't slide out of.

You could get a car cushion that slants. Put the thinner edge at the back of the chair and thicker part at the front. If the cushion slides, there is a rubber mat you can put under it.

I'm wondering if you put dad's feet on a step stool in front of his chair, might give him more stability. I'm not sure it's just a thought.

It depends on WHY he is sliding off the chair. If it's a wooden seat that's slippery a not slip chair pad or something as simple as a pad made from no slip rug pad can help. There are also moulded, saddle style chair pads designed to help people sit upright. A captain's style chair with arms will limit side movement as well as give him support standing and sitting.

As long as someone is with him while he’s in the kitchen, I would search a medical supply store for some type of vest that would secure him in the care. Could use a wheelchair. Or belt him to the chair, place bath towel between him & belt.

This is one, but there are definitely others, in different designs.

https://www.amazon.com/Vive-Wheelchair-Seatbelt-Adjustable-Prevention/dp/B0BFSV8YWB/ref=sr_1_3