Lolly pop symptoms and conditions

My 3 year old has been on Singulair off and on since he was 1. This last time he started in November of 2008 and began getting agitated, extremely moody, and having fits of anger by Christmas. I thought it was a phase until about 2 weeks ago when my son's doctor told me he thought it sounded like side effects of the Singulair. I took my son off the Singulair and it has been absolute anger fits every time he gets upset or does not get his way. These anger fits have lasted anywhere from 30 minutes to 3 hours (including one that was an hour and half at the doctor's office). The fits consist of screaming at the top of his lungs, biting, kicking, hitting, and yelling rude and crude statements. Once they start he does not know how to get out of the fit. Now that he is off Singulair, I just want to know how long the withdraws will last....Any suggestions of help with the fits or details on how long the withdraw will last would be appreciated.