Are you sure it's not the permanent tooth? It's a valid question, because I was 8 or younger when my 2nd molars were replaced by permanent ones and I and my parents don't remember when it happened (and my parents were certainly not negligent, especially as I was an only child). So I think in exceptional cases it's possible.

I don't think (at least, I've never heard of it) a dentist would choose to perform a root canal on a primary tooth, they would probably just extract it. Or at least the dentist should have presented you and your husband with the option of extraction, which would probably have been better if it is indeed a primary tooth.

You must keep him on a very healthy Diet (lots of fruits & vegetables). Low Sugar is crucial & no artificial sweetners. A good multi-vitamin & extra vitamin C would also help. Thank goodness it's a baby tooth!