There isn't really public transportation to Gillette (like you mentioned). It's kind of a pain to get to unless you drive (and even then it'sstill a pain). The traffic getting into the parking lot is always a mess and it's even worse leaving.
But if your son can get a ride from someone, doesn't mind tailgating for a few hours before (and after) the show, he'll have a ball.
It makes for a very long day, but if he's a huge fan, it will be a blast. I've done it over a dozen times and I enjoy it.
Hey, maybe I could give him a ride. We might actually be taking a party bus so none of us have to worry about driving.