Allergic reactions can be a lot worse at night, due, in part to normal, night-time, metabolic changes. To avoid Asthma worsening at night, your son must avoid as many allergens or irritants as possible during the evening. Things that don't really bother him during the day can certainly bother him at night. For instance, let's say that your son is mildly allergic to the gums in toothpaste. In the morning when he brushes his teeth, there could be absolutely no reaction at all to the toothpaste. But, at night, the tooth paste could set off a worse reaction.
So..... at night, it is much more important for him to stay away from chemicals, odors, pets, etc., even though those things did not bother him during the day.
However and unfortunately the bed is the worst place for allergens and irritants. It's kinda difficult to avoid the bed at night.
Does you son get worse before he goes to bed, or after he lies down in bed?