Self images 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?