I think depending on the severity of the MPI, some dairy can be tolerated because of how it's made, it's proteins would be broken down more during processing. Still, you will want to be weary. Often a slip up with F would not cause issues and lull me into a false sense of security thinking he's over it, but it can accumulate I think and cause problems when exposed in larger quantities. I'm not sure what the best course of action is for re-introducing dairy. I have heard that yogurt is most easily tolerated, again due to how it is processed, so maybe you could try that?