Flippers and AOs often have strong detents which I have to imagine would be equally painful for someone with bad arthritis. Have you considered a friction folder?
The Ohta pictured above is d2 steel and cocbolo and comes in under $100. He also makes a fancier version in a laminated steel. The exposed tang allows the knife to be opened with your thumb, off hand, leg, table, etc. Really anything you can press the tang against will open it.
If your pop's been using slippies forever he probably has good cutting technique so the lack of a lock shouldn't be an issue. The blade can then be closed in the same plethora of methods.
@knifeswapper happens to sell them, along with some very nice leather pocket slips on his site: collectorknives.net