I just have to wonder what your reaction would be (your gut instinct) if a mom at the school, who was an over-the-top volunteer, supermom, picked up a group of little girls in a stretch limo for a princess birthday party. Would you think of her as a pedophile? I think that (in general) it's really sad that the idea of a man with a bunch of young boys automatically makes people think dirty thoughts.
However, I totally agree with going with your gut instinct. Nature gave mothers hairs on the back of our necks for a reason.
But I would be very careful to evaluate your feelings (perhaps some jealousy that they did something REALLY cool and perhaps out of many people's financial league) before you pursue talking to the administration. They are likely going to want to know why you don't want you child to be with this boy, and if you haven't even talked to this parent to get to know him at all, you're going to be making, what to them, will be some VERY groundless accusations.
Schools don't just allow volunteers without some background checks. This guy has passed their test, obviously. Statistically, he's probably a sound guy. And if he is, and you make accusations, you could end up with at a minimum, egg on your face, and worse, a lawsuit.
I would say your best course of action is to pursue establishing school rules that volunteers are never alone with the kids (if it's not already there) and otherwise, just don't let your kid hang out with his after school hours.