The allergens may be spiking during the cat's heat. Does this cat spend more time in his bedroom than the other one? I would suggest you change your son to a different bed and see if he still suffers the same reaction.
Also note some info on dust mites:
A 6 year-old pillow can get one-tenth of its weight from mites, dead mites and mite droppings.
The average mattress can gain five pounds a year from human body ash. The dust mite feeds on dead skin scales that people shed every day. The dust mite will eat an average of 50 million skin scales that people shed every day. The average mite produces about 20 highly allergic fecal pellets per day, which is enough to equal their weight in just a few days.