Well, that is considered a negative number. I can tell you that I've met people here with sweat test numbers in the teens who did find two CF mutations with the genetic testing. Personally, I'd push for the full panel genetic test. You may have trouble getting your insurance co. to pay for the genetic testing with a negative sweat test number.
In the meantime, are they doing any treatment for your son? I think you posted about stool problems? Maybe he would benefit from taking enzymes?
I know you are struggling here, and I'm so sorry. I know how it feels to want to help your child feel better. To know in your heart that something just isn't right, and be told by doctors that it's just allergies and asthma. I sincerely hope that you can find the answers you need to help your son.
Stacey