Tell him to take a lesson - he's not going to get everything he needs from here, even though the comments are accurate. He's got some longtime and deeply ingrained bad muscle memory of balancing and riding on the inside ski. That's also why his turns are mostly longish. He can't round the turn because the less supporting outside ski will always diverge as the turn progresses and go straighter. The fact that he is so far back and inside dictates that outcome.
He may be a boomer, but the plateau that he is at now is not going to disappear without some fundamental changes in his skiing. His athleticism shows that he can likely still improve, so it will boil down to whether he is willing to do the work to improve, or he is happy where he is. If he chooses the latter, that would be a shame - why wouldn't he want to get better and have MORE fun skiing?
Here's what he needs to say to the instructor after he signs up for the lesson. "I need to learn to balance over the outside ski". After the instructor watches him make a few turns, I promise you there will be no argument. It's going to take a lot more than one lesson too - his habits are hard ones to break, and there are other bad things going on (hands is one) that he may find functional for his type of turn, but which will have to be unlearned in order for him to progress.
It really is about fundamentals - I would start him over with wedge turns on easy terrain. He probably wouldn't cotton to that, but the wedge is such a great teaching tool, and he could be taught the movement patterns that he would later use in parallel turns to transform his skiing. The beauty of the wedge is in how it allows you to isolate the movements you want to work on a stable platform, with the added luxury of time.