I completely agree with the other posters. And Lady Magoo's point about possible drug use is dead-on. I don't know why I didn't think of that, as dh can spot the meth look a mile away due to his line of work. At any rate, yes, meth can do a number on one's teeth. If that's not it, the lack of ordinary, reasonable hygiene is very concerning.
I can only imagine how heartbreaking this is for you & I'm so sorry you're going through it.