Unless its uber light utah pow, hes going to have to work a bit harder at first. Here are some basic tips to start with.
#1 "attack" the mountain. Actively ski, not passively float. Get his shoulders out over his toes, and keep the legs close together. On a fatter ski you want just within shoulder width apart, but on skinny skis you want a close together base. Don't let him ski like a pussy. You should be able to see him putting effort into every turn, pushing the snow. Don't let him lean too far forwards or get lazy and lean too far back.
#2 Steeper is better. The heavier/deeper the pow, the steeper you want to ski. This allows you to keep your shoulders pointed downhill and out over your toes. The flatter it is, the more leaning back has to be done to keep the tips from diving, especially on skinnier skis. Be able to identify the fall line and ski down that so you don't have to fight horizontal pull from gravity. Once he gets better this will matter less and come naturally.
#3 Be ready for a workout. Skiing pow well takes a lot of effort. Not much more than skiing a groomer with good form, but the pow takes away the ability to ski lazily that most people employ. Flexing, extension, tip and turn. Repeat. Any other method is going to suck, and this method will kick your ass if you don't do it on groomed runs regularly. This vid
shows the method on groomers, but you need to apply it to pow too. This step needs to be combined with #1.
#4 Start easy. Get him to learn this shit on a smaller pow day. 6" of med water content is great to get started. Dont make him learn on a 20" day or he will just get tired pissed and confused.
#5 Demo some pow/all mountain skis. Its usually pretty cheap and he can try out a non traditional camber ski. You still ski the same way, but it makes pow turn initiation and flotation much easier. It makes pow more fun and less work, but it isnt a must. However for a beginner pow skier it will make his life easier by 10x. I like to think good floaty skis start above 105mm waist, but with unique side cuts and cambers you can find a skinnier ski that still floats well.
#6 Know that all pow isnt created equal. almost all pow, even wet heavy stuff, can be fun to ski. But the wetter and heavier the harder it is to ski. It can also be too light, if its 12" of super dry on top of ice chunks hes probably going to sink down and just be skiing on ice chunks. Somewhere in between will be best. I see you live in Oregon, so you will probably encounter a vast majority of pow variations. Each breed of pow needs to be skied slightly differently, and trial and error is the best way to figure it out.