Do his keys travel much to actuate? It may also be actuation distance, rubber domes are sometimes shallow and easier to fumble even for those without Parkinson's.
I've only tried brown switches, but he may be ok with blues, they have the same actuation force as black but also a tactile bump and a click when actuated. Perhaps that would help? For more quiet pressing but less force, there is brown. Greens would be the strongest tactile bump switch for very difinitive presses.
I can't comment on the difference but I think any mechanical switch with a tactile bump before or at actuation will help.
buckling springs may work as they require a more definitive press but the keycaps may be smaller on many IBM style boards.
depending on the severity of the tremors, I would consider showing him big button keyboards. There is also this key blocking option that requires you to stick your finger through a square hole to press a key.
https://www.keyboardspecialists.co.uk/collections/accessorieskeyguardkeyboardsApparently it's for their big key keyboard;
https://www.keyboardspecialists.co.uk/products/k-ulxqw1I would start with a basic cherry mx blue, brown, or green board, see how he does. Might be just enough. I think bigger keycaps are better than narrow spaced caps, less chance to slip off the cap into the valley (undoubtedy how some mispressings happen).
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