Hi,
My son has asthma and so do I...and after breathing test after breathing test between the two of us, I have found out that when you have allergy induced or exercised induced asthma, the breathing test will only come up positive when you're having a flare up. I would suggest taking him to a pulmonologist at a children's hospital for a 2nd opinion on asthma. They can have him walk on a treadmill or something to get it to flare up and then do the breathing test. They will also test him for other allergies. There are also plenty of things he can be allergic to that they don't test for. I guess he could be having anxiety attacks which are giving him difficulties breathing. Although, I don't know what would cause an 11 yr old to have anxiety..but everyone's body reacts to things differently. I hope they find the problem and get it resolved quickly for him. Please keep us posted on him! Good luck!
Sandy