If he wants the bottle for another 6 months it won't hurt him. Bottle rot is from laying the kids in bed with a bottle in their mouth and it staying there all night dripping milk onto their little teeth. It doesn't allow the saliva to rinse the mouth out and the milk just eats away at the teeth.
If you brush his teeth and don't let him HOLD the bottle in his mouth all night then he can take a bottle as long as he wants it.
Think about it. We drink milk, coffee, pop, all kinds of stuff, don't run brush our teeth after every drink or bite of food all day long. And we don't get bottle rot. A drink is a drink is a drink whether it comes from a bottle or a tippy cup or a coffee cup.
When he is ready he'll hand over the bottle and be done.