Most foods, especially breakfast cereal, aren't fortified with calcium. My
son likes broccoli and spinach, which both have (relitively) high calcium.
Also Tuna. Other greens, like kale, collard, too. If your son can have
wheat, whole flours (the bran) has calcium, too. But to get even close to
the amount recommended (plus vit D for absorption), you'll probably have to
bite the bullet and go the vitamin route. I just found a chewable-(+D)
milk, soy, corn, yeast, and wheat free Calcium from Nature's plus. They're
spearmint flavored, and 250 mg Ca/tablet, so a child would take 2/day. Not
bad for adults, either.
Helen has a good list at http://non-dairy.org/nutrition.html

I'm off to spend and hr+ at the local grocery to find more foods/ eliminate
gluten foods for my kids. Daughter has nickel (corn, cans, tomato, etc) and
possibly oat allergy. Son has Soy, milk, peas (legumes), barley (now all
gluten- I'm keeping oats for a few months to test), eggs, nuts, peanut
allergies. I think I'll check in at the front desk so I don't have security
watching me.