I have heard of this yes, its called transient lactose intolerance. I would do a few weeks on lactose free milk, SMA do one called SMA LF, it is over 5 pounds for half the size of a regular tin though
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Maybe he would take that better than soya, its not too palatable xx
And grrrrrrrrr to the doc, ask for a second opinion and you might even get a script seeing as its a temporary thing. Also try and stick him on to a bland diet for couple weeks.
It WILL clear in its own time, but if you dont want to wait then lactose free milk is available. You can then reintroduce normal formula after couple of weeks
As I said do get a second opinion on the matter xx