2 yr olds nebulizer treatments?

My 2 yr old son has allergy induced Asthma. I have a nebulizer cause even though he does not have an attack from strenuous activity, he gets them as soon as his allergies flare up. The doctor told me to give the treatment as often as needed when he is wheezing but I don't know how often is too often. I have given him 4 treatments today, the last of which was 20 min ago and he is wheezing heavily again. I don't know if I can give him another one. I have called his doctor and am waiting on al call back but I want to know advice from parents while I am waiting. I a out of town right now and states away from the doctor. Please help if you can.
Forgot to say that it is Albuterol.