K.C.
I would suggest a blood test. My daughter is allergic to eggs. We had a very tough time. You can do an elimination diet, but it's difficult. She was a baby and was reacting to something that I was eating. I did the elimination diet and thought it was wheat. Later had the blood test and it was eggs. I had to take my daughter to her doctor to get a Rx. Then I had to take her to a lab. I would suggest taking another person who your son is comfortable with. It took 3 of us to hold down my daughter to take her blood. It sounds terrible, but she was younger. If it is milk there are many alternatives available. My daughter loves the almond chocolate milk. I get the unsweetened and add stevia instead of using sugar. Good luck!