Sorry, meant to explain that vitamin D helps in the absorption of calcium, so if you take the calcium with the D, the calcium gets absorbed and the blood levels remain stable.
I'm sure your son is on it.
Elaine
If you do not take calcium with vitamin D, you then run the risk of hypercalcemia (too much urine in the blood) and also may develop too much calcium in the urine.
Too much D with not enough calcium can also pre-dispose you to kidney stones (because of the hypercalcemia and too much calcium in the urine).
I am hoping your son's doctor told him that!
Elaine