D.W.
Your pediatrician is the best source of information on how much calcium he should be getting each day at his age. The human adult body can only process 500 mg at a time.
You can visit the website for Registered Dieticians: http://www.eatright.org/
They will list out all the foods (such as broccoli) that are high in calcium. And, they should also have information about how to prepare them (raw usually retains the most nutrients) to get as much of the desired nutrients as possible.