Wow, this must be so confusing for you. I will do my best too answer your questions, though I think the only way you will get real answers too suit you and your child, is by going back too your doctor and asking for his opinion on things.
The figures I have for sweat tests are:
If the concentration of chloride is over 60 mEq/L, the test is positive; 40-60 mEq/L is borderline; under 40 mEq/L is negative.
It is very possible that as your child is at the very top of the borderline range, it isn't CF. There is always like a 2% chance that the test isn't accurate on some people. I'm glad you're going for another sweat test though, as it may give a clearer result than the previous one.
I would say, showing no real major symptoms of CF, and with no genetic mutations that your son probably doesn't have CF. I've never actually heard of anyone having CF, but not been able too find at least one of the gene's. But then I'm not a doctor, only a patient.
My sister has loads of problems with her tonsils and throat infections, and ear infections and is asthmatic. Though if she did have CF, she would have at least one copy of Df508 muation-the most common. But, she hasn't. It's weird because she actually has more time off school than me and my brother (both of us have CF) ever had.
Good luck on the 1st, and hope things can be made clearer for you. Hope your little one gets better soon.