VCD and peak flow

My son is a long distance runner who was diagnosed with VCD last fall. He did not respond to the methacholine challenge as an asthmatic. His attacks occurred when running repeated sprints, but over the winter, spring and summer he has continued to train without any problems. He was found to be allergic to dust mites and grass, among others. He also had post nasal drip and reflux. He was on meds for both allergies and reflux through March. Now off meds and not seeing an allergist.

My question is if he is not having attacks of VCD, but has consistent peak flow reading of 400-450 at the age of 16, height of 70 inches, is there a potential problem to be addressed because of being at the low end of peak flow? Of course, we do not know what the peak flow would have been prior to the VCD. This fall, he has raced the same course under excellent conditions, but been slower by 30 seconds than his times last year before the VCD episodes, showing a lack of stamina in the final mile, despite the good training runs.

[Tammy2009]

I know that peak flow readings are determined by height, age and WEIGHT.  Looking at the normals for my height and age I should be greater than I am because I lower than the average weight.  

[runnermom831]

Do you have a source that also gives weight? I expect my son is average weight, at least among runners.