Hiya, my daughter was diagnosed lactose and soya intolerant at 12 weeks old, it was a real battle to get the docs to listen, They prescribed pregestamil for het which is a milk substitute that we fed and cooked with until she was 2 years old. She was a little small for her age but is catching up and now, nearly 5, is the picture of health and can take milk and soya. It's a relief to get the diagnosis because once their milk is changed they become so much more settled. Make sure his diet is varied and you screen his foods for stuff containing milk because you'd be suprised! And talk to your HV she will be able to give you some advice. Most children will grow out of it by the time they are 2.
If you've got any ?'s feel free to ask, I will try and answer them for u. XX