If test for cystic fibrosis came back boderline how do you know if he does have it?

Clinical course. If any test fo CF is borderline including sweat chloride, the subsequent clinical course will point to the correct dx.
Monitoring. Careful monitoring will be needed, by an experienced CF physician. The sweat chloride testing is used in combination with other tests, and with history taking and a clinical exam, to make a diagnosis. The sweat chloride testing is often repeated more than once.