M.M.
The fact is that the human body was never designed to drink cows milk which is why so many people are lactose intolerant. If the doctor is content with him making up the calcium in a tums, I'd probably settle for that. Pushing the issue (especially if he's onto you) will only make him fight you more just because he can (hence the terrible 2s). My suggestion is to give him the tums for a while and in several months try to reintroduce milk in subtler ways.