http://www.ssa.gov/disability/professionals/bluebook/ChildhoodListings.htm
Check out the above website for impairment listing. THe one for CF is under 103.00 (I believe it is 103.04) on RESPIRATORY. IF your child also has any digetstive issues, look under the 105.00. There is nothing specific there for CF but if you child meets, or even almost meets any of the digestive listings, that might be enough, especially couppled with any other CF symptoms listed under 103.04.
In December we won Mark's (My husband) CF case and appeal after 27 months and 4 denials. DO NOT REAPPLY, APPEAL APPEAL APPEAL. That way if your son is found disabled down the road, you get back pay. Whereas if you keep reapplying, you loose that initial application date.
If you have any questions, please feel free to email me. division902@hotmail.com Even though my husband is clearly not a child, the impairment listings are almost the same for adults and children (save for the numbers, weights...) so I can send you some spread sheets and information we used which helped win Mark's case.