I ride, my wife skiis. If we feel like riding the same terrain, we
stick together...if not, then we do our own thing! It doesn't really
matter what we ride the mountain with, just that we're enjoying the
mountain.

What's stopping your son from skiing the terrain park? If he's not
interested in it on skiis, why would he be interested in it on a board?
Perhaps he just needs to find friends that like to ride the same
terrain he does. Switching to snowboarding just to ride the terrain
park doesn't sound like it will make him happy, IMO.

Eric