Welcome to Epic!
First off 9 years old is old enough to learn to ride. Large people can also successfully learn to ride, but instructors with successful experience doing this are few and far between. If you attempt this, you should plan on SLOW progress and consider getting private lessons from a pro with a good referral. Heavy people tend to struggle more learning to ride than learning to ski.
With respect to learning to ski or ride, your son's progress will depend on prior experience and where you go. If your son has prior experience with sideways sports like skateboarding, surfing or wakeboarding, he will find snowboarding easier to learn. If your son has experience skating, rollerblading or water skiing, he will find skiing easier to learn. With modern equipment and teaching methods, the learning curve for both sports is similar. At some resorts learning to ski or ride may be easier because of the terrain, the rental gear or the experience of the instructors and the teaching methods they use. In general, though, prior experience will be the determining factor.
In general skiers have a harder time learning to ride than riders learning to ski, but once you've reached the stage where you let the edges do the work, learning is greatly accelerated in either sport and learning the second discipline is accelerated also. It's been my experience that learning a second displine works best when you ease into it (e.g. spend part of a day on the new gear, then the rest of the day on old gear), although some people fall immediately in love with the other sport and quit the first one.
It sounds like you'll be better off starting with skiing. At 9, it won't be long before you'll be too slow to keep up and you'll have a head start relearning to ski. Once he gets good enough on skis to dump you, you can reset the clock a little by letting him learn to ride too. With modern skis, skiing is a lot easier than it used to be. You're going to have a lot of fun.