I can't really help you with what your situation is, but can just share that we are in 'cf limbo' with one of our kids, too. My son has the same genetic results as your child- one df508 mutation. His sweat test results were 48 and 54, so a solid borderline. Appt with urologist was completely 'inconclusive' on whether or not he has his vas deferens. His growth has slowed over the last several years but nothing like the growth failure that led to Emily's dx; he has chronic sinus issues and a new breathing issue when exposed to a couple specific triggers (a reactive airway type of thing) ...but again, nothing like his sister.
Emily is accommodating: she has the df508 and while her 2nd mutation is unknown, her sweat test was very high (108) so there was no doubting her dx, esp when combined with her symptoms, which are much more pronounced than her brother's. When it comes to our son, had his sister not been in the picture I am not sure what they'd be thinking of him or if it would even be a consideration for him at all. Genetically we cannot know if he's a symptomatic carrier or if he has mild (to date) symptoms of cf. His last sweat test was about 1.5yr ago so I don't know if we should do another; Emily's cf dr recommended trying to get coverage for nasal potential testing and he found a center that will do this for Shawn (they want to do a whole workup on him.) I hope this yields us some valuable info because this living in 'maybe, maybe not' world is driving us nuts.
I am glad to see how healthy your child is... hope that continues and that somehow you get more answers.