I dont see why he can't dive dry, most 10 yr olds make adults look stupid when they play them on video games or such (multi task loading multiplied !!!)..... If the kid is smart, and listens to directions and respects that what he is doing is dangerous. I'll bet he'll pick it up pretty quick, in fact I'd bet he'd pick it up faster than most adults would. So what if he starts his diving career as dry ??? he wont know any better, and if your instructor is a good one, he'll do just fine.
make sure you take a LONG course..... no 2 weekend cert crap, when i got my OW I was in 8'th grade, it was a 3 mos class... at least back then they were less a c-card mill, and cared more about the training. ppl nowadays don't want to spend that kind of time. if you can't find a place that will do a longer course, hire a private instructer to teach, you'll spend more, but you will learn ALOT more than you will in a quickie class.