Turns out an old filling has caused a major crack in his tooth, and the good news was that they weren't going to have to do a root canal, but needed to get him in pronto to deal with the problem tooth and crown it. He goes in tomorrow, and he's a little nervous, understandably, as he's never had one before.
They gave me plenty of nitrous for mine, and I know they'll be kind to him, but I'm going with him tomorrow just in case he's woozy.