M.S.
I suggest watering it down as much as he will let you, and for at least a little while, let him have it that way. With the milk so watered down, the amount he will have left behind on his teeth is no big deal.
You could also try leaving him with two sippy cups... One with only 4 ounces of milk, and another full of water. If he drinks the water afterward, it will rinse his teeth.
Or, what we did with our son is we cuddled with him while he drank his milk, then still in the dark of his bedroom, brushed his teeth real quick and then laid him down. We were eventually able to get him to drink it in the kitchen as a snack before getting ready for bed.
You are asking for two big things from him... to learn to put himself to sleep, AND to give up his milk before bed. See if you can find a way to help him do just one at a time.