Is it a barium swallow? My son had severe reflux. He vomited constantly from birth to around 10 months. Although he did gain weight. No one ever believes us when we say how sick he was as it doesn't sound possible but it was. He had various blood tests for allergies and also the barium swallow. All that they did was starve him for a morning no food or drink which was a lot easier than it sounds, then he had an early morning appointment at the xray dep where he had to drink a white drink then lay down on a table while they took photos of the drink going through his upper digestive system. It wasn't pleasant for Nathan purely because he has a major fear of doctors/hospitals from the blood tests. They found nothing abnormal.
He suddenly just stopped being sick at around 10 months old. Seriously he went from constantly vomiting to just nothing. We don't know why he was so sick. On a blood test they found an immune deficiency but they don't think it was related to the sickness. He never lost weight as he was on Neocate which is ok as a food substitute, has your pead mentioned this? It is worth asking.
Reflux can be caused by Pyloric Stenosis but what we were told was that you would know by a few weeks old if they have that as they are seriously poorly with it. Also a loose muscle at the bottom of the throat can cause reflux which is more common especially in premies, and it just gets better as they get older but they can do an op to strenthen it. Reflux is also more common in boys and hereditary.
My husband had it just like my son when he was a baby but my younger son was lucky and didn't have it. Anyhoo I have babbled enough, I hope it all goes ok and your lo gets better soon,it is very hard having a child with sever reflux and you can only know how bad when you have been through it
Just want to add, my son was never seemingly in pain either but they need the antacid to prevent damage to the throat inside as the constant acid can really cause damage.