Jessica, I have known plenty of babies for which it is both. Even an underdeveloped GI tract will usually allow breast milk unless there is a true allergy. They baby might just have gastroparesis or slow emptying. Depends.
Noah was 6 lbs when he came home at 3 months and gained pretty slowly. Preeemies can be small. Even now, Grace and Noah are still tiny. Oh well.
Two ideas for Reece:
1. Add coconut oil to everything. I mean EVERYTHING, including formula. Coconut oil is awesome with the calories and surprisingly easily tolerated. I am sure some kids are allergic to coconut oil, but in all the tubies I know, I haven't found one. Flax oil is another good oil, but has higher allergy rates. Ditto avocado oil.
2. I think formula is way harder to digest than real food, especially the crazy "elemental" formulas. But if he's not vomitting, I wouldn't sweat it too much. It's the reflux issue that is maddening. She could ask the pediatrician for duocal which is a calorie increaser.
We had a feed schedule much like Reece's and it was hard. But it came with meds too. Noah's bottles were terrible looking because of the iron and the everything else. But he was trooper and drank it anyhow.
I'd also say her pediatric specialist is well-intentioned but adding extra stress to mom is bullshit. Preemies are preemies. If they are growing then don't get hung up on the curve. Does this specialist have lots of preemie patients?