This would be so rare as to be a topic for a case report in a journal.
Nothing is impossible, but I would never accept such a diagnosis without
a biopsy.  Even then, it is not always easy to tell.

There is no reason to wean a child for ulcerative colitis.
Nevertheless, it is possible that there is so protein in the milk
causing an allergic colitis. It might be worthwhile looking at some
possibilities.

Also, the fact is that some babies have had these symptoms, and have
been "cured" by paying more attention to getting the baby more hindmilk
(express an ounce before feeding, keep the baby longer on first side
with breast compression increasing hindmilk intake, even two feedings
per side).  In theory the lactose malabsorption does not cause blood,
but why couldn't it?  And we have had some cures.

What really has to be looked at is the general health of the child.  If
the child is growing, happy, and otherwise in good health, the blood in
itself is not a great worry, though it does cause worry.  He obviously
needs to be followed.

Jack Newman, MD, FRCPC