I've worked with older shooters before that have had some sorta tremors/shakes.
I'm not sure if it's the same thing, but for the most part it's been nothing more than another obstacle to overcome.
NPOA should work - he's just not going to see the rock-solid front sight that we see.
If he's going to be using the isometric pressure as you describe, it'll be a matter of being consistent and relaxing as best as possible. Think - Offhand. In the offhand position there is always some 'wobble' and it's difficult to find and maintain a NPOA because you have to use muscle to stand. But you work to eliminate as many as possible and minimize your wobble by firing the shot the moment you pause your breath as well as taking rests in between.
As far as the trigger work goes - that'll be a question you might have to find out there. I'm not sure how this would affect actual motor skills. But I suspect there won't be an issue unless he's got a hair trigger...
If he's got good eyes, I wouldn't recommend a scope - and if he uses one, it ought not be more than 3x as the amplified movement would likely break the mental game.
Present the instruction as normal - Work with him, if something needs adjustment - IAO. No big deal.
I'm not even sure I'd go right into the isometric pressure thing, without a huge background of how/why it works for him.
Sounds a lot like the push/pull pistol technique. If you've got access to a laser trainer - put him on that and work with him 'till the movement is minimized. If not, just watch his front sight.
One older gentleman I worked with who had the 'shakes' or whatever it ought to be called - had them pretty bad. Made it real difficult to trust his NPOA. I did notice one thing, and I'm not sure what he was doing, but his front sight would bounce like crazy up until the moment he was firing the shot - somehow he would stop it - BANG - then back to bouncing. Didn't think at the time to ask how he did it. Anyway, as soon as I got him to turn his scope down, trust his NPOA and follow through, he was shooting good groups - once he got stage 2 & 3 down, he scored Rifleman.
Now in this instance, he did have some sort of mental control over the tremors - so I'm not sure how relevant this is.
Perhaps there's someone here that knows more about this specific condition. But if all else fails, work with him, do the best you can and let us know how what worked and what didn't.