Yogurt, cheese, cottage cheese... tons of Ca++ and fat rich sources out there aside from milk (do NOT use low-fat yougurt, or low-fat cheese -whole milk products only :)- they need those fats for brain development, but do not need the carbs from the filler used in low-fat products ).
What we did though, was switch to chocolate milk. Not the kind you buy at the store... chocolate syrup + whole milk + shake shake shake. Voila. That way you can control how much chocolate goes into it. We'd use about 1-2 tablespoons per 20oz... vs about 1/2 cup per 20oz which is apx what's in storebought chocolate milk. The syrup was more than enough to change the color & flavor, which is what he really wanted. When we first switched though, and when we were hiding liquid medicine in it, we'd load up on the chocolate. And then, easy peasy, gradually lower the amount we put in.
Just checking though... are you serving the milk warm or cold? My kiddo wouldn't touch cold milk until he was almost 3. So he had "hot chocolate" (actually, he called it "warm" chocolate, and was quite empathetic about it not being hot ;). He'd eat cold yogurt/ cheese/ juice/ etc., but milk needed to be warm.