Dairy is one of the primary culprits in terms of reflux - either in formula or when a breastfeeding mother consumes dairy. I learned this the hard way as my first son sounds like yours. I was off dairy with my second son, who was almost 9 lbs at birth and quickly gained 1 lb/week for the first month and then 1/2 lb per week for the next 5 months - his weight gain finally slowed down when he was 6 mos. This is contrasted with my first son who was similar in size as yours at close to a year.
Also, gluten intolerance/celiac can be a primary cause of poor weight gain. We didn't learn that this was impacting my older son until he was 10. At age 2, the doctor said he would be petite (I am, but dad is 6'3"). Once my son started the GF diet, he grew 4 inches per year and gained 20 lbs per year for 4+ years. At 15, he's now 6'4".
I can't urge you strongly enough to consider both the dairy and gluten correlations with reflux. Also, the prevacid, etc. reduce stomach acid and thus make digestion of nutrients far less effective.
You can get both the dairy and gluten tests done on your own: www.enterolab.com
I used to recommend the blood tests that you could ask your doctor to run but most doctors are still clueless about this and rarely run the right tests even when asked. And, when they do run the right tests, they don't know how to interpret. So, you can save yourself alot of grief and money by running the tests on your own. Since it is a dietary change, you really don't need a doctor involved, however, you should share any results with your doctor. Once I had my sons tested, the pediatrician was fully accepting of the diagnosis (I'm hoping this opened his eyes to this situation). We have a family friend whose a pediatrician who was very interested to understand the symptoms that my boys had, so he could start looking for this in his patients. As far as endoscopies, they are often inconclusive at this stage as it is rare for there to be enough damage for GI docs to pick this up - it takes nearly 100% damage to the intestines - of which there are many feet in the body - for an endoscopy to be accurate.
The enterolab stool test is by far the most accurate available - the lab has all the necessary certifications and then some. It was started by a top GI doctor in Dallas - he felt it was his mission to share this testing immediately rather than waiting the 10-20 years to have this accepted as standard practice. Many progressive docs run this test. And, slowly even the most conventional of GI docs (including the top celiac one) are coming around the the significant value of this testing. Testing for celiac via endoscopy is a 50 year old way to diagnose celiac and it likely will be phased out with the next 5 years.
Celiac is the MOST highly underdiagnosed condition in this country (only about 6% of the people with this condition have been diagnosed), which is why it is something that should be tested in ANY child with poor weight gain, reflux, any tummy troubles, rashes, etc.
Also, folks with older kids should consider this testing for any child diagnosed with learning disorders, in particular ADD/ADHD. There is research showing that both the gluten and dairy proteins have significant impact on the brain. There is a top psychiatrist that does brain imaging and he has seen these impacts.