I have a few students with hemi paresis/plegia. One, who is late 50's and had a stroke 5-6 years ago has found that those rear-entry boots that got re-introduced recently have made getting his boots on and off much easier. I would not hesitate if you have a good bootfitter, to work on lifts and canting to try to get him as neutral as possible, without having to do extra work with his bad leg. If you can find a bootfitter that works with a lot of adaptive students, all the better.
My student struggles with the mitten issue as well. I suggested he get a wool mitten for a base layer, plus a shell mitten with a big gauntlet (like the ones pictured above for an over-glove). I'd done that once when I broke a finger and still needed to ski. He skis with a slider (you've probably seen them if you've been to adaptive programs - they look like an old lady walker with skis instead of tennis balls). But it doesn't sound like your son needs/uses any adaptive equipment.
I assume your son doesn't have the grip strength to use an outrigger in his left hand. That would be a cool thing if it worked. It might also be interesting to experiment with one on his right side to assist with turn initiation since he probably can't generate pressure and angles with his left leg. There's also a side benefit to having outriggers. He probably won't like them because they'll make him stand out on the hill. But standing out isn't always a bad thing. If other skiers give him a little extra room, that's awesome. He probably doesn't make turns like non-disabled skiers. That's understandable, but it isn't always predictable. And when you make turns unpredictably, you're more likely to get bumped into. If he could 4-track, he might really like it.
Does he carry a pole in his left hand? If so, and you can get the LEKI system to work, that might be helpful. If you bring him out to Tahoe, contact me and we'll set up something at Achieve Tahoe. We have some awesome people and tons of adaptive equipment. But I'm guessing your adaptive school does also.
As for skis. If he doesn't venture off-piste or on harder black runs, I'd go with a good front-side ski. Wider isn't going to help him and could cause unnecessary stress on his knee. He's probably not able to generate big angles with his compromised leg, so anything that makes it harder to get the ski on edge (like wider ski) is not going to do him any favors. My guess is he mostly skids his turns, so a ski with a bit of early rise with help make it so he's less likely to catch an edge and can pivot skis easier. And they don't need to be long. I routinely teach on a pair of 158s and I'm 5'10" and 200lbs. When I'm not teaching I can still ski those skis plenty hard.
Keep up the great work. Its great that he's out there and enjoying skiing. See you on the hill.
Here's a pic of me and my student who had a stroke.