Could you maybe just try cutting out milk?
You'd have to be careful with this as soy milk is not as calorific as cow's milk and I know there was a case where a couple only fed their child soy milk and the baby died but I think that was the only thing they fed to their child and they didn't breast feed and the child was very underweight.
This site has some info about feeding babies without milk. They've just started selling in the UK real cow's milk without lactose in it although I'm not sure if it's available where you are. I don't know if it is the lactose though or something else in the milk that causes it though. Lots of people here say their
KP got better after cutting out milk (but not me).