Asthma is not an occasional thing, it's an everyday thing. Your son's lungs are constantly inflammed even though you don't see the symptoms all the time.
1. You need to get him with a children's lung specialist.
2. Your son needs a daily maintance medication to prevent these attacks. Then you won't need to go through these monthly issues.
3. You need to "de-allergize" his world as much as possible.
4. Get rid of or cover up with allergy poof material anything that can harbor dust mites and dander.
5. Keep the pets out of his bedroom.
6. Get a hepa filter for his bedroom.
7. Wash his bedsheets in hot water and hypoallergenic detergent and fabric softener.
8. If anyone smokes in your home have them smoke only outside. It's better if they quit because the odor clings to them, but if it's hard to, try to limit your son's exposure to it. Your son is already having to deal with irritated lungs he doesn't need the added irritant. Perfume and air freshener are also irritants.
I hope these things help out. I've suffered from asthma for over 20 years. It's managable as long as you limit the triggers and take your daily medicine and keep your emergency meds on hand.