What do you do about this?  I was just reading in baby 411 that you absolutely have to brush your babies teeth after the last feeding of the night, even to wake the baby up to do it.  I do not agree with what the book says about sleep, as I will continue to nurse to sleep and nurse at night, but I am concerned about his little teeth.                
                             
        
Re: bedsharing/nursing through the night and teeth cleaning.
I thought the same thing turns out breast-milk isn't the same as giving a bottle. and bf babies get less cavities.
https://curetoothdecay.com/Baby_Bottle_Tooth_Decay/breast_milk_cavities.htm
https://naturalchild.org/guest/linda_folden_palmer3.html
Little Rose is 2 1/2.
Here's another one:
https://www.kellymom.com/bf/older-baby/tooth-decay.html
Dental carries and breastfeeding are sometimes related. My friend whose 3 year old still nurses to sleep has tooth decay, so for the last 18 months, she's used xylitol wipes to clean her teeth after she's fallen asleep. She brushes before nursing.
Something like this:
https://www.opentip.com/Toys-Kids-Baby/Wholesale-Tooth-Tissues-Dental-Wipes-For-Baby-Toddlers-Pack-p-1578570.html
I wouldn't be too worried about it at 9 months old, as long as you're cleaning off his teeth before bed (I never did more than wipe with a cloth before they had 8+ teeth).