I'm so sorry you are going through so much trying to help your child. I've been there. My son was 15 yrs. old before he was diagnosed. I always knew there was something more than asthma and severe allergies, but I hit brick walls much like you have.
If you can get your child to a CF clinic in your area, please do. I think you have to test for it. Also, have any of his doctors mentioned enzymes? With the weight gain problems, I think they should consider trying them.
I really hope you find the answers you need to help your child. Keep us posted.
Stacey