Hi Eva - just saw this. Don't know if it's too late to respond or not. I found skiing to be easier to learn than snowboarding. I learned to ski pretty well in one day, but it took longer for me to get a decent feel for snowboarding (and I did skateboard as a kid).
But, that being said, I enjoy snowboarding more - and once you get past the beginner stage, I think snowboarding is actually easier. I'd say the best thing is to let your son choose. You might decide that skiing would be smarter since he might well learn it faster and have more success on the slopes on this particular trip (probably true), but if he sees all the young people out there on snowboards having fun, he may regret not trying it.
Some time back, I posted some tips for learning to snowboard. They're at the link below. I hope they help if he tries snowboarding...
I hope he has a great time, regardless!