Do you have a peak flow meter? If not I'd get one. When my 5 year old gets sick I check her breathing several times a day to see if its the "bug" that's going around or if its her breathing. We up her daily breathing treatments to twice a day when shes sick. she uses pulmicort.
Yes, I would give the treatment.
Many times breathing tests take a log time to reflect what is going on in the lungs. Be pro-active here. I would start doing treatment every 3 - 4 hours.
There is a really nasty bug going around here, a para-flu virus. It isn't the flu, but it acts like the flu. It might be a good idea to go ahead and take him to the doctor.
Take care and I pray that he gets to feeling better soon.