I use an Orbweaver, and definitely like that over a keyboard, so a Tartarus could be right up his alley.
If his right arm has a decent amount of dexterity, you could consider a trackball. He may want to put it on a slope to angle it for better accessibility.
Seems this is a repeat thread. The person that published the video said this is the trackball he uses: https://www.amazon.co.uk/dp/B000HEW0KW
Quoting him:
"The ball is different because I am using the trackball from a super old Logitech trackball mouse (Can't remember the name but it isn't produced any more and I threw it out years ago)"
Nice caps. I have seen plenty of beige-colored trackballs when I was browsing for one but they are all vintage things on ebay with no scroll option. I'm not sure if you could mod them..
The silver orbit is not discontinued on Amazon, but beware that it doesn't have a scroll ring, although I'm sure there's a ball-scroll mod in guides somewhere.
And you can always opt for the silver Expert.
I love my trackball mouse
At top of my head, with similar formats, I can think of:
The allmighty Gameball:
https://www.gamingtrackball.com/
Kensigton Orbit:
https://www.amazon.com/Kensington-Orbit-Trackball-Mouse-K64327F/dp/B000HEW0KW/ref=sr\_1\_14?crid=85UQKHD3WD9X&keywords=trackball&qid=1649898048&rnid=2528832011&s=electronics&sprefix=trackball%2Caps%2C193&sr=1-14
Kensington Orbit with Scroll Ring:
https://www.amazon.com/Kensington-Orbit-Trackball-Scroll-K72337US/dp/B002OOWB3O/ref=sr\_1\_2?crid=OE01N3WNAH5U&keywords=kensington+orbit&qid=1649898730&sprefix=kensington+orbit%2Caps%2C226&sr=8-2
Adesso's IMouse T40:
https://www.adesso.com/product/imouse-t40-adesso-2-4-ghz-wireless-4-button-desktop-trackball/
that know seems to be called KODAK IMOUSE Q40
Adesso iMouse T1:
https://www.ergodirect.com/product\_info.php?products\_id=16357
Which are all more or less similar to Logitech Marble.
Now, with center ball but with a format not very similar to Marble, there is also:
Kensington Expert
https://www.kensington.com/pt-br/p/produtos/controle/trackballs/expert-mouse-trackball-sem-fio/
and it's wired version:
https://www.kensington.com/pt-br/p/produtos/controle/trackballs/trackball-usb-expert-mouse/
The "Ferrari of the trackballs" Kensigton Slimblade:
https://www.kensington.com/pt-br/p/produtos/controle/trackballs/slimblade-tball-kensington/
But I would strongly advise you to give a chance to Elecom Huge:
https://www.amazon.com/ELECOM-M-HT1DRBK-Wireless-Trackball-Mouse/dp/B0735584RM
It's the best in the whole world in my view.
Everything after the 10-digit product number is just tacking data:
https://www.amazon.co.uk/Kensington-Orbit-Trackball-ergonomic-ambidextrous/dp/B000HEW0KW
Here is the mouse: http://www.amazon.co.uk/dp/B000HEW0KW
The ball is different because I am using the trackball from a super old Logitech trackball mouse (Can't remember the name but it isn't produced any more and I threw it out years ago)