My son took singulair for years with no problems, so we started using it for my daughter with absolutely no preconceived ideas about it.
My daughter took it when she was 12. It was horrible! She became so depressed, and was crying, fearful, upset, you-name-it. She wasn't in puberty, so that wasn't it. I totally believe it was the singulair. I told the doctor who prescribed it, and she said to stop it immediately--that she's heard of this type of reaction.
It was so bad that my daughter wanted to give up ballet, soccer, everything because she was afraid to leave the house. We didn't mind her giving it up if she did it because she was sick of it, but not because she was scared. (She started dancing lessons at 3, but didn't get serious until she was 4. She is now 17 and studies all day, about nine hours a day
and loves it, so dancing is very important).
Anyway, at 12, after being on singulair, she was so fearful that I couldn't leave her side!! She was terrified something was going to happen to me or her, and I had to go to every ballet lesson! I stayed in waiting room, but she even worried about the future when she would go on tour with the company. I had to assure her constantly that I would go on tour with her, following in a car. She didn't even want to try out for her beloved Nutcracker. Her sisters and I literally had to put her in the car to go to the audition, where she was crying and crying. (She got a good part anyway, but if she hadn't tried out she would have gotten no part at all.)
She wouldn't even go to her friends' homes--the ones she'd played with for years. She was living in a spirit of fear. We had many people praying over her.
Bottom line: she got off the singulair, and went back to her normal self. She's now been on tour many times....without me.

A real-life friend experienced something very similar with singulair.
If you google it, you'll find many situations like what I've described. Since I hadn't googled it before using it, I was totally unprepared.
Does he have asthma with his allergies? How old is he? I take Sudafed decongestant when necessary (generic, behind the counter--have to ask the pharmacist for it). Probably not recommended for kids.