My daughter's experience is somewhat similar to your son's. She had failure to thrive at 4 months and was exclusively breast fed. She was sent for a sweat test and an upper GI. It was determined that she had reflux and her sweat test was a 34 (not borderline but on the high side for an infant). Fast forward a few months and since I was still concerned about the possibility of CF, despite a normal fecal elastase test and a negative CF genetic screen the doctor ordered the full Ambry genetic test. The results came in 2 days ago that she is negative for mutations but has something called a 5T variant that acts like a mutation. So, in essence she is a carrier and that variant is what causes her sweat test to be on the high side of normal. So, it is quite possible that your son could just be a carrier!