I'm not an instructor, but here is what I see. I would not describe his skiing as being backseat. He has a residual form of the "little kid power wedge" and he has to be backseat to get enough leverage on his wedge. Maybe getting more forward would force him to straighten his skis, but I think it is more likely that straightening his skis would allow him to get out of the backseat.
Hopefully somebody with real experience teaching kids will chime in here, and confirm or deny my diagnosis. And suggest a path forward.
The important thing is to keep him liking skiing. If he has any friends that ski, take them along -- that was huge for my son. They will motivate each other much better than you ever could, and have more fun doing it. With a few obvious exceptions, kids are not very interested in technique. My son skied sporadically as a young child, and pretty regularly around your son's age, but really didn't get good till high school. Now he is a young adult, an excellent skier, and a good ski buddy.
Oh yeah, hate to play to an EpicSki stereotype, but are you sure his boots aren't too big? When they were around 8 years old, one of my son's friends skied kind of like your son. We discovered that his boots (bought used, with a "hoiw do those feel" method) were a lot too big. When that happens, you have to push your feet (not your shins) forward into the boots to have any leverage on them, and that means backseat.