My pediatric dentist and I had a long talk about this, and if you google it you will also see the facts. Holding the bottle all night long in the mouth is what causes the teeth to get weak and decay. Saliva naturally rinses the mouth, if the milk is continuously dripping onto the teeth for extended amounts of time then the saliva cannot rinse it off and protect the teeth.
How many people do you know that eat all day and drink stuff all day and don't run to the bathroom and brush their teeth. It's the same concept. If you hold a piece of hard candy in your mouth all day, every day, the teeth will eventually start breaking down and get decay.
I have had dentists say that if you have weak enamel there is nothing to do to prevent decay. It will happen. If the family genetics are for strong enamel there is not much that can weaken it.
Our little grandson, 4 now, stayed on the bottle until he was 2-2 1/2 and has zero cavities. His big sis stopped the bottle much earlier and had lots of cavities. I only found out she had cavities because she fell and broke her front tooth and had to see a pediatric dentist to have it capped and that's when we found out about the cavities.
If you are taking good care of these teeth then having a bottle at night is going to do little to hurt them. As he gets older you can wean him away from wanting it. But I still held our little guy to get him to sleep for a long time. I miss those times now.