Have you considered that he might be lactose intolerant? I thought I was allergic to Diary and I was told in the hospital I was just Lactose Intelerant. For the first time in many years I just take a tablet, there are several brands, and no problem!
You can also get drops. Apparently it replaces the enzyme many of us are missing to break down the protien in Dairy products. I have doing this since January and I can eat even pizza and milkshakes anything. It is the best thing to have come out of this.