Hi there, so sorry to hear about your son and I know the worry!
My oldest son when he was 9, had a sweat chloride test and the first one came out at 40ish and the second one came out at 60, and when tested for the type of mutation he was carrying (takes so long!) they only tested him for the first 1,000 mutations and he had two mutations not matching, and the Doctor at childrens diagnosed him as a carrier with symptoms. He does have lung issues and it also affects his GI tract terribly, not to mention he is now 16 and weighs 80lbs. My daughter was also tested and her sweat chloride was at 80 both times, and she is now 12 and while she only weighs 60lbs, her lung capacity is good and she participates in cheerleading.
Best wishes to you and I hope and pray that your son will be okay.