Poor weight gain is a concern, but why stop breast milk. Thickened feeds are
not the only way to treat. Would need more info to answer intelligently, but
most babies who have reflux do not lose weight.
I would not be satisfied that they have found the cause yet. Also it doesn't
explain the drop in hemoglobin, unless he also has a GI bleed.
Reflux is not an absorption problem, more of a volume/calorie problem.
(usually just a laundry problem). ABM instead of breast milk would be just
substituting one fluid for another. Only advantage is that it can be measured
more easily. Using expressed breast milk, or test weights(if you have the
proper equipment) could overcome that advantage.