SIX ounces a day!!!
At this age toddlers need 500mg of calcium per day - there is about 300 mg in 8oz of milk.
Here is a lit of some typical calcium containing foods so you can see if he gets enough calcium from other sources --
Milk, whole or low-fat 1 cup 300 mg
Soy or Rice Milk 1 cup 300 mg
White beans 1/2 cup 113 mg
Instant Oatmeal 1 cup 100 mg
Broccoli, cooked 1/2 cup 35 mg
Broccoli, raw 1 cup 35 mg
Cheddar cheese 1.5 oz 300 mg
Yogurt, low-fat 8 oz 300 mg
Orange juice, calcium fortified 1 cup 350 mg
Orange, medium 1 40-50 mg
Sweet potatoes, mashed 1/2 cup 44 mg
But, there is also the additional fat in whole milk that is not in orange juice, broccoli, etc.
I don't think there is an "age" to drop the bedtime milk...I STILL to this day have a glass of milk before bed each night! If he doesn't want it, then I would offer it with his lunch and dinner instead. At this age lots of parents move to milk three or four times a day in smaller quantities so it is more like a drink with a meal rather than as a "feed" - so 4oz in the morning, lunch and dinner, kwim?