I would definitely but the kid's pass since technically he is 9 when you purchase the pass. Just because he will turn 10 after you buy it should have no bearing on it. Everyone has a birthday sometime within the year of an annual pass.
Yeah, and I've never seen anyone get asked for a birth cetificate at the gate to prove your kid is the right age for the kid's pass. Plus there are some 6 year olds I've seen that could pass for a 10 year old.