Not wanting to be too critical, but I have a 'Helping Hands' item that has the two clamps and the central magnifying glass and I almost never use it, because it really isn't always that 'helpful'.
I myself am disabled, and while I have full use of both hands per se, my disability renders my energy levels almost non existent. That, and using my hands for anything really physical (just using a screwdriver for instance) can if I do it for just a bit too long, make them hurt considerably.
End result, I deal with a regular daily dose of frustration over what I once could do, but now can not.
The trick is to limit or severe restrict actions that slap the disabled person in the face with their disability.
That's why the challenge in this case is so high, as just one hand is going to be almost impossible to defeat. Not totally, but very close to it.
I have done it all, ie every sector of model making, cars, trucks, ships, planes, tanks and figures. But in the end, you rarely escape a need for two hands, the fine motor response required to hold something in fingers, as opposed to unfeeling clamps, while the other hand attaches a part to what is being held carefully in the opposing hand.
Just about everything needs assembly. Almost impossible to escape it.
Parts clean up, models simply don't come without parts clean up.
But you CAN do figures that lack assembly. Airfix has a very complete array of subjects from all time periods and all settings. Frontier, Napoleonics, WW2, Modern. They would only require a well thought out system to hold the item while he painted it from all manner of angles. And as most of these figures come with a base, you get something to clamp onto which is all the difference. The base can be clamped without fear of damage, because he can always attach to a better scenic base later. Or he could approach it as part of the initial work. A wooden block screwed onto a holder from underneath would give a very secure mounting.
Then it's all about patiently positioning, then painting what is available from the current angle, and just keep repeating that process.
But then you get to how to open a bottle of paint.
Regardless of his solution, he WILL need a partner, someone that simply is ok with being an active participant in his hobby. But I am assuming he will require someone to aid him in non hobby activities as well, so I suppose it is not going to be such a stretch that he will have someone available to do no account tasks like open or close up a bottle of paint.
I think the key to the whole deal, will be to sell him on the value of what he chooses to do.
I have sat here staring at all of my models, and try as I might, I can't picture doing even the most featureless, bland looking tank with one hand and a helping hand grabber. Not 100% impossible, but perhaps too likely too be too frustrating that I would feel good about being too positive with suggestions.
In addition to the Airfix figures that are made of a soft plastic and come in a 1 piece item for the most part, there is also the fantasy metal miniatures popular with the role gaming crowd. Not very military admittedly, but I need to some times remember, the world is not just tanks :) They are also non assembly in most cases and really just a challenge to paint. Then it becomes all about eyesight, and not so much about having two hands.
I hope this helps, and remember, it's coming from someone that lives with very real daily frustration.
It will be a hard sell, getting him to believe.