Welcome to HealingWell. I hope you are able to find some help from the link Razzle gave you. He really seems to be knowledgeable about food allergies. I have several food allergies, one of them being milk. I know as a small child I never drank milk because it made me sick. I'm not sure what I did as a baby. I can still remember in the first grade the teacher would make me take a carton of milk and I would put a straw in it and pretend to drink it. I had told her it made me sick, but she would not listen. I guess I never thought to have my parents talk to the teacher. They certainly didn't try to make me drink milk at home.
I always knew milk made me sick, but it wasn't until I was an adult and had the RAST test that I knew I was truly allergic to milk. I don't get the side effects your son gets, so I'm afraid I can't help you there.
I do hope you find your answers very soon. I know you hate to see your child sick. Please keep us posted.