[recycled sixtie]

To me we start off as children and end up as children. Being an "orphan" ie having no parents or mil or fil left we have gone through the whole nine yards. Definitely in the 90 year old range they need direction. Don't feel guilty doing whatever it takes to ground him. Disable the car or get his licence revoked as the above says. For peace of mind it will bring on a large sigh of relief. You do not want your fil hurting anybody and /or himself driving. It takes courage on your part but I believe it is worth it.
You are the boss now....
Cheers, Guy