It could be a side effect he couldn't predict or prevent. I had the weirdest nerve pain in my jaw / teeth. I was thinking trigeminal neuralgia (and still do); one of my consultants (dentist, maybe) thought it might be referred pain from a bad tooth. Nobody could figure it out, and none of us could make it feel better (which was all I really cared about at that point, not even my neurologists). It had to have been a side effect of the treatments, whatever it was. Oddly, once I lost all my teeth due to the radiation and the subsequent removal surgery, that particular pain went away. So either the referred pain was correct and we removed the problem tooth along with the rest ... or it was a TMJ thing, and I can't clench my jaws anymore because there's nothing to clench at night, LOL, Or it is some kind of neuralgia, but we still managed to remove some source. Anyway, yes, again a lot about me ... but using it as an example. It was NOT something that could have been predicted or avoided. Unless it really was a tooth gone bad and better hygiene might have helped, which I doubt in my case since I lost them all and probably isn't in your husband's case since it hasn't been that long (but is definitely worth checking!).
If it is a nerve thing, there are meds that can help with that. I react badly to most of them, which is why my neurologists weren't as much help as they might have been. (They don't handle patients well who don't fit typical molds.) Most folks do find relief if that's what the problem is. Worth looking into.
But dentist first, since it seems tied to the specific tooth. If they're experienced with cancer treatments and side effects they should know where to point you next. Does he have and use the fluoride trays? I think you can still use them after treatment (no, I did not, alas), and that would probably help preserve some of the other teeth from succumbing to damage. Nerve/tooth pain can be a great wake-up call even to the stubborn!
*Not a medical expert, just a well-practiced patient, LOL. My official caveat.
