Asthma management plan symptoms and conditions

What's the safe replacement drug for Singulair? I took my son off it before and he had his worst asthma attack ever. I see all the same symptoms in my son who's 9 and has been on it for 4 years now. It's so scary. We're off to the doctors to see what can be done. My Pharmacist has never indicated that there was ever a change in the side effects to Singulair. Thank you for all your helpful postings.