You're most important task is ensuring a successful day on snow. Regardless of which sport you choose, make sure your son is well fed, well hydrated, warm, comfortable and having fun. Don't assume that he can go all day. Be prepared to stay till last run if he's having the time of his life, but also be prepared to you suddenly needing to go in early if you detect the "pooped" signs. Also make sure that there is no pressure. If he does not like one, let him try the other. If he does not like either be prepared to offer other alternatives. Be aware that the national average for first timers converting into regular participants is about 16%. In a family situation, the odds your son will enjoy a snow sport is higher but still not guaranteed.
When snowboarding first started to become mainstream, the learning curve was faster on a board than on skis. With the advent of shaped skis, the learning curve for skiing caught up. The biggest factor for learning curve currently is previous sports experience. If someone has experience with forward facing sliding sports like inline skating, ice skating or water skiing then they will tend to pick up skiing quicker. If someone has experience with sideways sliding sports like wake boarding,surfing and skateboarding, then snowboarding will be easier to learn. The easier a sport is to learn, the more likely one will enjoy it. Although not relevant to your personal situation, the conventional wisdom is that skiing is easier for children under age 7 to learn. This is because children develop physically from the center out and snowboarding requires more ankle movement than skiing to manipulate the edge changes. Burton is working hard on equipment design and teaching techniques to address this. I have successfully taught 3 year old children to ride, but this requires more hands on teaching and careful terrain management than when teaching young first time skiers.
Helmets should be mandated by parents for either sport. Wrist guards are an important piece of safety equipment for first time riders. Sprained wrists are more common than broken wrists, but are usually avoided with proper lessons. Modern teaching techniques minimize "slamming". The most common trigger for wrist injuries (and tailbone injuries) in snowboarding is catching the downhill edge. Modern beginner boards have a high edge bevel (e.g. 4 degrees) to minimize the risk of catching the downhill edge.. Losing an edge to the uphill side of the board rarely causes a hard fall.
Although many schools do not teach how to fall in lessons, I prefer to teach first time riders the "secret sound" method of falling. Practice this method by getting on your knees, then making fists with each hand, then crossing your arms over your chest, then crashing onto your chest. As you make contact with the ground say the secret sound: "OOOMPH!" That makes you exhale on contact (don't land and then say oomph or say oomph and then land) and lessens the impact. The secret part of this technique is that if you try to remember everything as you are falling, you won't get past make a fist before making contact. If you think "oomph" as you're falling you can get all 3 steps done in time to prevent injury.You can practice this at home on the lawn before your trip.