That's a bad ass little kid! A true champion the way he takes spills and just gets back up.
The main reason they teach kids skiing at a younger age than snowboarding is that snowboarding requires more fine motor skills and core strength to make the board turn.
I would work on his core strength with whatever activities at home you can figure out. Something like jumping jacks will work both the core and hone in on some fine motor control to make them clean.
Other than that, just try to get the kid making some actual turns. He seems to like sliding down on his heel edge, so get him on his toes and sliding down while looking over his shoulder.