In the morning he was totally side sliding down the mountain. We had a bit of a 'discussion' over that and by the end of the day while I won't say he was carving he definitely can go up on heel edge and go cross slope without slipping.
Should I adjust the 'camber?' on the bindings a bit to force his knees to bend? Right now I have them straight up.
Side slipping. He's 9 and has been out 5x. Could be because he's more comfortable with that at this point, especially considering one is facing down the mountain/fall line when side slipping (heel). Beginners of all ages find comfort in facing down the fall line.
Bindings. I think you mean the highback? Personally I don't like any forward lean on my highbacks and ride them straight/neutral. I think the best way to bend your knees is to actually bend your knees. When you do, it's not just the knees that flex but also the joints of the ankles and hips. If he has the tendency to ride with straight legs, likely his ankles and hips are locking a bit too. Forcing the knees to bend with forward lean won't engage the other joints as much as if he bends his knees consciously. But some love fwd lean on their bindings so there's that.
Disclaimer: I'm not an instructor. I don't have kids. I don't know that much about kids (apart from common sense and a nephew whom I love), teaching kids to snowboard, or teaching anyone to snowboard for that matter.
Oh, I recall some old threads on here about making a game out of it to learn the basics. Might want to do a search on that.