Good question, Chris.
Just for perspective, I've been snowboarding for around 15/16 years. I also alpine and telemark ski. The last time I was on a wakeboard was on several occasions a few summers ago (it was also my first experience on a wake board). The first thing I though of was when carving quick turns, was this was the best off snow practice I could get. They couldn't get me to stop, I was having so much fun. I think there are several cross-over moves and techniques. The only thing that was drastically different was being pulled by the boat. The tendency was to pitch my overall stance back a bit, where in snowboarding I generally try to keep perpendicular with the snow's surface, so I end up actually feeling a bit forward.
Your son will have to adjust for the "lean back" when trying snowboarding. Just an FYI, it may take some time to get up and moving around on the snowboard, so I'd highly recommend some professional instruction for at least his first couple of days on the snow. Then continued instruction as he progresses and wants to explore more of the mountain and the different areas of fun in snowboarding (trees, bumps, park, etc.) I think your son will have fun with snowboarding and can translate many of the wakeboarding moves to the frozen water. Where are you planning on taking him for the first time snowboard experience?
Think of it as not just taking extra equipment, but loosing a lot of heavy equipment (2 skis w/ bindings, two boots, two poles) in exchange for one piece of relatively light equipment (still have the boots though). Hope this helps.