Singulair Bad Dreams, Terrible Twos, Naps, Agressive, Wit

My son was prescribed singulair shortly after he turned 2 and has been on it now for approximately 3 months. At first we just thought that he was going through the "terrible twos" but are now starting to think otherwise. He has gone from a child who had daily naps to no sleeps at all during the day and is often still awake at 10pm at night. He is waking during the night after having what we assume are bad dreams, not recognizing us and lashing out. When awake during the day he is destructive, aggressive and extremely naughty. We are almost at our wit's end with him - at first I did not link it to his medication - it was friends who commented on how he has changed since beginning to take it. We are not sure where to go with our fears as our GP is fairly dismissive regarding this drug and we have to admit it has helped his asthma but at what cost!