Would your doctor be willing to try reflux medicine? Our son started growing when he started that. He's still not big, but he's gone from being below the 2nd percentile to being at the 19th percentile. Yay!!! Took 3 years, but we'll take it. I'd try that before tests. The tests are invasive. Our son was a silent refluxer -- in otherwords, he had no signs like spitting up, etc. Tests has since shown he is a serious refluxer! Serious enough that had he not suddenly started growing this last year, he would have had surgery last Fall.
Check out the "Great Taste, No Pain" diet, too. My son and daughter who both have reflux eat like champs on this diet. The food is delicious, fresh, and healthy for everyone. It has made a big difference for both of my kids. My daughter no longer takes medicine, and my son's stomach doesn't hurt all the time any more. It also has helped my husband and especially my stomach. I never went to the doctor to get medicine because I didn't want the side effects, but I no longer has any problems. GL!