Not unusual for this to happen at all.
My son probably started with asthma when he was 3-4 y/o. When he was about 5, the pediatrician finally used the word "asthma" with me. Even then, he just had some wheezing and I was given the albuterol inhalers. I was not really educated about it and no one treated it like it was a big deal. Then, I put my son in a new daycare. The woman had a lot of animals and my son's wheezing and "allergies" became worse. Now, we have two dogs ourselves that don't seem to bother my son at all. He's also been tested for cat/dog dander and was negative. However, this woman was a breeder and I guess just the sheer amount of dander in her house was too much for him. After 3 months of chronic lung inflammation, he got a cold. Three days later he was in the ER with an oxygen level of about 88% and we could not resolve it. He spent 5 days in the hospital. Since then, we started going to a pediatric lung specialist (my son is now 8). They have educated us thoroughly and armed us with an arsenal of preventive meds (Flovent, Singulair, etc). It has been rough but we've really got him under control. If they haven't been treating your stepson's asthma aggressively enough, this could have very well been the kicker for the attack--especially since the pollen counts are outrageous just about everywhere. Have your husband get very educated and get the child medicated so that this doesn't happen again. Your stepson's oxygen level was VERY bad--that definitely was a bad attack.
Christine