Destructive path symptoms and conditions

My son is 12 and has been taking Singulair for the last ten months...he has been getting moody and upset for no good reason over the last 3 months and I put it down to "hormones" starting to kick in. He has now gone to feeling very sad and has even told me last night that he has thought about killing himself several times and feels he can't go on, or cope and feels lost and lonely or unwanted and in the way. He is confused and does not understand what is happening to him. I have just realized it may be the Singulair and will be taking him off it immediately. I'm hoping it is this as if it isn't I feel totally helpless as to how to help my child. Please say a prayer for him. He used to be happy go lucky.