Bronchiolitis is usually caused by a viral infection, most commonly respiratory syncytial virus (RSV
infections are responsible for more than half of all cases of bronchiolitis
and are most widespread in the winter and early spring.
It sounds like the relationship of bronchiolitis and asthma is still being studied. In the meantime, I would work to ease the other factors, such as exposure to second-hand cigarette smoke, that he might be exposed to. Good luck!