[red-beard]

To hold the carving edge, the feet need to be locked to the boots. On a toe side turn, the toes should be digging into the bottom of the boot like a claw. On a heel side turn, the toes need to not just extend, but need to be "pulled" up to the top of the boot.

To help "visualize" this, I encouraged my students to mimic these maneuvers with their hands. Further, they do the hand jive as they snowboard, at least until it is natural.

Finally, look at the board itself. If he is snowboarding on hard pack, you need to have sharp edges, properly beveled, or the board will not hold the hill.

If he is more focused towards all around the mountain, not terrain parks and half-pipe, he might consider race bindings and hard boots. You end up with much better edge control and hence carving. I use a carving board with hard-boots. If they still made rear-entry ski boots, I'd still be using ski boots.

I suggest lessons, but not too many place have intermediate lessons for carving. Usually, those lessons are focused on the half-pipe and terrain park. Maybe find someone for downhill racing/boarder-Cross training.