We have a daughter who is allergic to milk (like, she can die -- not just get an upset stomach) so we've been down this road. Her pediatrician suggests soy products ("milk", yogurt, etc.) and to supplement the calcium, TUMS. Also lots of broccoli and other green leafy veggies, which, fortunately, she still likes. So far, so good.

I am skeptical of the claim that milk contains things that leach the calcium out; that's more likely caused by other things in your diet (caffeine is known to have this effect, for example).