C.J.
Everyone says children need lots of milk to build their bones, but what they forget to say is that COW's milk is mostly a calcium that human bodies can't absorb (they also don't mention that you need magnesium and vitamin D to fully absorb calcium!).
If you're worried about the amount of calcium he's taking in, one of the most calcium-rich foods is sesame seeds. Tahini is a GREAT way to get calcium into a child's diet. 1 teaspoon of it has 129mg of calcium. An entire CUP of whole milk has 236. You can mix tahini half and half with peanut butter and kids don't even notice the difference (or mix it into hummas, or on soft bread by itself. There are a million ways to eat it!).
I wouldn't worry about your little guy only drinking two cups a day. He should be fine. Are you still breastfeeding (or formula feeding) him at all?
I personally would be a little more concerned that he's drinking watered down juice when he's thirsty. The watered down juice should be a treat or a snack, and he should be drinking plain water when he's thirsty. Otherwise he'll get into the habit of having to have "flavored" things to drink. :)
Just my two cents. :)