Hi S., I know what you're going through. My oldest had dental issues early on too. If finances allow, it couldn't hurt to get another opinion. It could offer more information to aid in your decision, or at least a little piece of mind. With regard to milk, I wouldn't place too much emphasis on that - a balanced diet whenever possible should be enough.
Besides, it's my experience that genetics play a huge part in these kind of things. My oldest daughter has had almost 4 cavities in her preschool years and my youngest, not a one. I have friends with twins and the same thing - one with lots and the other nothing. Having said that, I am obsessive about their dental hygene because of it. They use a pre-rinse, floss and after rinse and I get a sealant after each cleaning. That seems to have helped, no cavities for the past two years!
I, too, would be very nervous about anesthesia and that's a decision that only you can make. My advice about that is find out who is doing it. I would expect/demand a very experienced anesthesiologist.
I know that whatever you decide will be the best for you and your son. Good luck!