But to do away totally with what he was happy with just because we said "that's weird", that's alarming, in my head. I guess I have an allergic reaction to the notion of someone allowing themselves to be told what to do when they're happy with where they're at.
Well, he came asking....already with the suspicion that his method might be a "bad thing", and it is certainly not a "good thing".
I can't speak about his mixes because we have not heard them, but having done that sort of thing (and as you've also confirmed from your experiences) the more you stack or "reduce down" track after track....the worse it gets. Sometimes also, what people think sounds "good" may not be based on any proper reference or other methods to compare against.
Granted, DAWs give him some option to go back, unlike tape days.....but it's not the best way to do it, and I'm confident if he approached mixing from the way most people do it....he would see why and gain from it....but if it becomes somewhat of a crutch doing it his way, he will have a harder time later on facing a mix with 30-40 tracks in it...etc.
It's not about telling him what to do....rather pointing out why he should.

Teachers do it all day, every day....help break bad habits, to help someone improve. So there's nothing alarming about it, and I don't always buy into the notion that "anything goes" if you "think" it's good.
In the long run, it can handicap you when it's time for you to expand your horizons.
I'm just suggesting that he provide a logical reason why he's doing the way he is, which I don't think he has, and his "reason" might be more of a "seems OK to me" kind of thing.
We are giving him the option and reasons/examples of why the other way is better in the long run.
We are empowering him with options that will help him improve.
I wish I had more of that input and direction when I was first starting out....but back in those days, it was ALL trial-n-error, and the amount of time and effort on doing something the wrong/hard way didn't always pay dividends, but yeah, sometimes it's also good for someone to learn some things by trial & error.
It's like the never ending mixing with headphones questions....some of those guys also say they are happy with their mixes so do they really need monitors....?....YES.