If you haven't had this discussion with him already (and you think you can comfortably have this chat), he may already have some ideas that he hasn't thought to mention.
Otherwise, one suggestion would be controllers for quadriplegics. There are a few options out there, afaik most are controlled with your mouth but there are also gaze-tracking options. I believe the XBox accessibility controller is compatible with some of them.
I could have that talk with him. But i don't want to start it with no options or no suggestions.
Him and his wife don't have it easy ( they used to go walking in the mountains a lot, so the declining mobility in recent months are ... Depressing for them, although they don't really complain. I just notice it in the conversations when it comes up)
Can't really make any hardware recommendation, but as this post is about games, I would suggest trying out more puzzle like games like "The Witness" or "Outer Wilds". Despite being single player, I played both of them in tandem with my wife and I feel like it was a much better experience than playing alone as there was always something one of us would have missed or would get stuck on.
Well, it's game hardware for "disabled", so this is one of the best threads similar to my question. Where people could have the same interest or know about something already.
If he can comfortably move his head, might I suggest something VR related? Most of the first VR games that came out for mobile (i.e. using your phone as a VR screen) couldn't utilise a controller so they used head movement for controlling things. Usually you had e small crosshair in the middle with which you can select things by simply holding a position. I sadly can't remember the names of the games I tried but you might find them (or newer ones for newer headsets) through google. I know it's not a complete substitute but if they miss mountain walks, there are VR experiences (often videos) which simulate that. Might be interesting to try. I'm not aware of it but there might even be multiplayer ones, so that his wife can join him.
Otherwise, one suggestion would be controllers for quadriplegics. There are a few options out there, afaik most are controlled with your mouth but there are also gaze-tracking options. I believe the XBox accessibility controller is compatible with some of them.