I'm MPI and I can't drink goats' milk either (unless it's raw, in which case I can drink cow or goat). So if he's truly MPI, that could have been a trigger.
Yeah, formula is nice because you KNOW what's in it and it's (theoretically, anyway) consistent all the time. But in many ways that's also the beauty of breast milk. And there's no guarantee that it won't take several different tries of formula in the event he reacts to something else in the formula that you don't know about yet.