Does your son like nuts? They are a great source of calcium, just a little handful each day of either almonds, walnuts, cashews. Be careful of peanuts, mold is a huge problem with them. I'm not sure of their calcium content either.
Calcium fortified orange juice poses a problem as the citrus blocks the absorbtion of the calcium from my understanding in the same way that chocolate does. These chocolate calcium chews and calcium fortified OJ just boggles my mind...