Interesting, unfortunate case.
From what you say this baby is taking 155kcal/kg/day based on birth weight and assuming an avg 20kcal/oz (.67kcal/cc) in breastmilk.
The baby's suck and lack of development are worrisome.
Has this baby had a metabloic workup? Somethings to check include  urine and electrolytes at the same time, urine and blood for organic acids and amino acids. Also Neogen labs in Pittsburg does a metabolic profile of 30+ diseases using filter paper.
Does this baby have loose stools especially with blood or white cells as you question milk allergy?
Email me privately if you need the number for Neogen labs. a lot of hospitals do the test as routine newborn screening as it takes only a few drops of blood.
Any test weights done around breastfeeding?
-Rob