If not a g 17/19...........
What about a PX4 or an M&P with or with out thumb safety. Both have large slides that offer a lot of surface area that we find many can manipulate them easily.
A long time ago, in a state far, far away I worked at Cabelas in the gun department. We were asked this question a lot. After trying every semi auto under the glass and showing them to a lot of potential customers...I learned the easiest semi auto for elderly and small frame women to work the slide on was a Glock 9mm.
CHW2021, Can you tell me what your mother shoots? I have a 75 year old very frail mother in law who could not pull the 10lb trigger on a Taurus pt22 and the glock 19 was too big for her hand.
Just to add... I would let him rack the slide on a few autos and see if that's an option.
My GF lacks the grip strength to rack my Glock 19. This gun has >10k on the recoil spring and is the easiest auto to rack in my opinion. But she has no problem racking a brand new M&P.40!
2 students at my last CCW class used full size Ruger 9mm's to qualify with. Not great for concealment but I was surprised as to how easy they were to manipulate. Just a thought.
Like mentioned above, take him to Jim's, Laf Shooters, etc and spend some time "playing" with each model to see what works best for him. Good luck.
If racking the slide is an issue the slide is far easier to rack on a Walther .380 than any pistol mentioned in this thread. Super easy and super smooth.
If racking the slide is the primary consideration then this video might interest you. I have no experience with this gun but ran across this video when looking for something else.