Positive reinforcement symptoms and conditions

I have two sons on Singulair, my three old was suspended today from pre-school for bad behavior. Screaming, biting, scratching, and hitting have been going on for the last few months. But worse the past two weeks. He has always been active and played like a boy, but he has been getting violent. We have tried positive reinforcement, praising good behavior both verbally and with rewards, I have tried time-out, being grounded, not being able to participate in activities, no t.v. and old fashioned butt spanking. All of this with no positive results. I am not looking for an excuse for my child's behavior, but could Singulair really be the cause of his aggression?

My son has been taking singulair for three years--since he was two. It seemed to really help control his asthma (cough variant). He has recently become extremely violent--especially in school or with his baby brother. I did not realize singulair could be the problem. I also noticed he has been very moody and antisocial for the last year. We have tried extra attention, positive reinforcement, reduced and restricted tv/video viewing, modified diets, punishments--nothing seems to help the aggression. The only thing left we did not think about is the singulair. Any recommendations for taking him off? Is cold turkey ok, or better to decrease the mg (1/2 pill, then 1/4 pill, etc.) until he is off?