Failure to Thrive and Feeding Issues - Please Help

My son Austin is currently 7 months old and weighs about 14 pounds.  He was diagnosed with Failure to Thrive at 2 months and has been running under 3% for the past five months.  I became pregnant through IVF at 34 and was therefore considered high-risk and was then diagnosed with gestational diabetes and anemia and monitored heavily towards the end of my pregnancy.  Austin was born at 6lb, 2 oz and was only fed with breast milk for 3 weeks until the pandemic hit and my milk dried up.  From there, his growth began to decline.  After begging his doctors to take our concerns seriously when they were scared to see anyone, we finally got in for an appt and they realized he was actually losing weight and began scheduling multiple appts for tests and trying various different formulas.  We were tested for CF (negative), and multiple stomach issues (fat malabsorption), but ultimately the current diagnosis is reflux and milk protein allergy.  He is currently on a daily antacid rx twice a day, taking Similac Alimentum formula and then we are supplementing with probiotic drops, avocado oil several times a day and adding rice cereal to several bottles a day.  We also give infant Mylicon as needed.  We were told to add in baby food two months ago so we have been feeding him Gerber baby food stage 2 3-4 times a day.  His issue has been he is such a sweet, wonderful, loving happy baby but he has a bad tummy.  He refuses bottles throughout the day and requires multiple attempts and then takes 2-4 ounces at a time despite all of our efforts so he gets at most 23 oz total.  We were last told to try Duocal but he vomited it everywhere once and then another time was inconsolable all afternoon.  I finally bought Benecalorie but have now been told by the doctor that it is for older babies.  To top it off, dealing with his doctors has been a nightmare.  The first pediatrician didn't do anything, the next one finally saw us and immediately referred us to a GI specialist because of Austin's weight but then the GI specialist stopped running tests, would only run 1 at a time and never performed any scans.  The GI doctor also takes over a week to call us back after each call (no in-person visits) so we don't know what to do or how to improve Austin's weight for extended periods of time.  Please let me know what you are giving your babies at this age to increase their calories - different formulas, additions, baby foods?  I read on another blog about Scandishake, Pediasure or coconut milk formula and have heard something about goat's milk but don't know what ages of babies these apply to?  I am so grateful for whatever information I receive and hopeful that you wonderful women can help.  Thank you!  Austin's Hopeful Mommy