I'm currently trying to pin down what ds is reacting too. He's OK with goats milk products, cows milk live yoghurt and (I think) cows milk butter with lactic acid added to it. We've been using a no lactose cows milk, which he seemed OK with (it has some lactose in it), but when we tried a low lactose cows milk, he got terrible diahorrea and stomach pains.
Anyway, in my search for what is actually in milk, I've just found this article, which makes interesting reading. I think I'll have to have a go at reading it without ds on my lap though, as it's heavy going!