This I am very experienced with. DS was cronic with croup and now has asthma. Around 4 is when it starts to present, so if it is asthma you will know. DD had eczema really bad and then had REAL bad asthma, and now only gets it on and off. DS had no allergies, real bad croup and now has asthma that seems to be pretty persistent where as DD's will only come up if there is a sudden change in weather. I would go by the Dr's advice and wait and see iykwim. If it is asthma the sooner you get him on a preventer the better, but if he's on predmix he should be ok. DS atm is on predmix, seretide and ventolin the poor kid. And he's going through another bad bout after a bad one about a week ago