Dear M.,
You've gotten some great advice here already, so I'm not going to repeat that. I just want to mention a couple of things that others haven't said.
First and foremost, it's unlikely that lactose is the culprit at your son's age. The problem is more likely to be milk *protein.* That's important because lactose-free formula will *not* help. He really needs to be off all forms of dairy (not only cow's milk products but also goat and sheep's milk).
Second -- okay I have to repeat something others have said, since it's such a good point -- the dairy lobby in the United States is very powerful. They -- the corporate interests that make a profit from milk -- are really behind the message that milk is essential to a child's diet and that it's the only source of calcium out there. Not true. Leafy greens, flax seed, fish oil, are all fine substitutes, and people who use these sources of calcium instead of milk have much lower rates of obesity, among other problems.
What I make for my son (whom at almost 4, still can't tolerate dairy) are "green smoothies." Calcium-fortified OJ, half a mango, and a handful of spinach. He gets one in the morning and one before bed, plus a liquid calcium supplement. He's a tall, healthy little boy who just about never gets sick.
To get the nondairy calcium complement just right, though, I really recommend consulting with a holistic nutritionist or a naturopathic physician. There *is* a cost to this, but you probably only have to do one consult, and it will save you from long-term anxiety.
Good luck!
Mira