Baby brother symptoms and conditions

My son is almost 6 yrs old. He has been taking Singular for approx 3 years, in addition to Claritin and Nasonex. This is all for his allergy to mold. His doctor started him on these meds after a 6 month episode with sinus infections leading to asthma-like symptoms. We have not noticed anything unusual about his behavior until approx 3 months ago. He started waking up every night (2-3 hours after going to bed) with bad dreams. At first he would not fully awaken and fall back asleep immediately. After a month or so of this happening most nights, he had two nights in a row where he woke up screaming and was inconsolable for about 30 minutes. We initially thought it was night terrors, but he was not scared or fearful, he was just upset/angry. He would hit the bed repeatedly and yell. He didn't know what he was upset about but would talk about how he thought we hated him, he was dumb, etc. After about 30 min he would 'snap out of it' and return to his normal self and go back to sleep easily.
At first we thought these were night terror brought on by a recent fever, but he had another episode last night. He has also been falling apart at the littlest things and getting quite violent (for a 6 yr old). He was sent to his room yesterday evening because he was complaining about the dinner his mom made (lots of complaining these days..). He had a total melt down an started throwing things and hitting the door so hard he put a hole in it.
Some of this behavior I would chalk up to normal kid stuff, new baby brother, etc. But I am suspicious about the negative thoughts, bad dreams, and instant melt downs... I stopped giving him Singular today and we will see what happens. God bless you all as we figure these things out...

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?