I searched for your previous post to get more information about what you posted last time. I see you are in the UK and not the US so I'm not familiar with how your medical system works, but I'd say keep asking the doctor for more clarification.
It still sounds to me like, yes CF is very possible for your son, based on his symptoms and borderline sweat test results. Many people on here have normal sweat test results but do have genetically confirmed CF. Usually borderline sweat tests and CF symptoms is enough for a diagnosis or at the very least being given the opportunity to benefit from the latest CF treatments.
A couple of questions here:
1) Do you know how many genes they tested him for? I know here in the US it is quite common to start off testing with only 98 mutations -- this to me is pointless as there are now approximately 1,800 known CF genes so anything less than testing for all of them is a waste of time -- you will not know unless you test for them all.
2) Is the doctor treating him "as if he has CF" but just not marking his medical records as a CF diagnosis? Just want to know if he is getting the best treatment and care even if he isn't diagnosed with CF. What does your doctor say is the reason why he will not diagnose him with CF -- is it because the sweat tests are in the borderline range?
3) Did the doctor do any fecal fat testing to see if he is pancreatic sufficient or not? If he is having weight gain and digestive issues he is very likely pancreatic insufficient and would benefit from being given enzymes with his food. Have you gone over this type of stuff with the doctor? What does he say about adding enzymes?
Best of luck - please keep us posted.