Arthritic hands may find it hard to perform the one handed opening technique, rendering the OH functions more or less useless. I would actually suggest a SOG as the implements are easily accessible despite being inside, the compound leverage would allow him a firm grip and the tension of pretty well everything can be adjusted to suit his abilities.
If you want to stick with Leatherman, I would suggest something like the Kick/Fuse/Blast series as they are well equipped, decently priced and the the implements aren't on a backspring so they also won't be difficult to fold out.
Def