I think you really have to work with the pediatrician on this when you have an infant. There's a lot more in breast milk than calcium, first of all. Why have you singled out this one element? Has he been shown to have a deficiency? What have you already discussed with the doctor about foods that contain calcium?
If you single out one vitamin or mineral and try to load up on that, you can cause more harm than good. I work in food science, and while there is no question that American diets are significantly deprived (and virtually no experts, including the AMA, think you can get everything you need by "eating right" anymore), I get very concerned when someone focuses on a particular compound as needing a boost. I only recommend a comprehensive supplement (no warning labels, no standard supermarket vitamins), never something that upsets the balance by being heavy on one or two things. A good example is "Vitamin Water" - one flavor has Vitamin C and something else, another has Vitamin A and something else....big marketing scam and a whole lot of expense for what cannot be absorbed because it's not complete. And it can be horrible for one's health.
If you can tell us more about what your child eats/doesn't eat (not just his likes but what the doctor has said not to give him)? Has there been any discussion of formula or any use of it so far? Lots of foods have calcium in them. Others have Vitamin A. Others have the B vitamins...and so on. Work on variety of textures, colors and flavors. Water is great - no juice at all is a good rule!
Anyway, if you elaborate more, it will help identify why calcium is a concern to you but not your doctor.
And if you find you rarely agree with your doctor, get another one.