Well, unfortunately not much alone.
A 48 is solidly 'borderline' which means that it can't be used to make a decision either way. (Borderline is 40-60).
Was it done at an accredited CF center? If not then the test is unfortunately not worth much, as the admistering of the test can affect results greatly.
Assuming it was done at a CF center, and your son is showing other symptoms, then I would expect either 1) a repeat sweat test or more likely 2) a genetic test to determine if he has any of the known mutations that cause CF.
I hope you can gain more insight, and in the interim (and beyond if it is CF) feel free to ask any questions you might have.