You can get calcium from various other foods, for example:
Broccoli, Chinese cabbage, collars greens, kale, turnip greens
Oysters, salmon (with bones- could blend I guess), shrimp, sardines
Molasses, tofu
If you think he is ok on processed milk then there is also
Ice cream, ice milk, yogurt, processed cheese, sour cream
I guess if you tried favouring the milk it may taste worse as its already flavoured.