I had the same problem with my son who is now 4 and the problems started when he was around 2. I was worried and asked the doctor and she said he could be lactose intolerance or just simply that milk is hard on his stomach. She told me to switch to soy milk. That helped him soooo much! As far as being lactose intolerance he does fine with other dairy products so I don't believe that was the problem I hope this can help you at all.
Take care and best wishes -Mopeybean :)
Does this happen when he has other dairy products? Like ice cream or cheese, etc.? If so, it sounds like he may be lactose intolerant. This is a pretty common thing, and fortunately most dairy products are made in a lactose free form. Try him on some lactose free milk or soy milk and see if it helps him.
He may have an allergy to diary products. Is it just milk, or any dairy products like cheese, yogurt, ice cream, etc? If so, he most certainly has a dairy allergy. If it is only milk, it could be the kind of milk. If you use whole milk, you may need to switch to skim and see how he reacts.