You can always wait to give it to your son until you can talk with the pediatrician for more information on it.
My son has allergic rhinitis, too. It kept getting worse and worse, starting around 4/5 years old... until at age 7, he was sniffling/sneezing and congested in his sinuses all the time. I should have invested in Kleenex (or Kimberly Clark or whatever). It was awful.
We asked our family doctor about it and he said, yep it's allergic rhinitis. Son had the crinkle on top of his nose that is a telltale sign (constant wiping the nose gives them a crinkle). This was in 2005. After a few weeks, I noticed that my son started complaining of his stomach hurting shortly after going to bed each night. Nighttime was when he took the singulair. I got out the bottle and read the side effects list: stomach pain was there. So I called the doc, stopped giving him the sinuglair, and put him on a Nasonex. It helped, but still didn't solve his issues.
It wasn't until much later that I read all the horror stories about singulair. I seem to recall some associations with depression and suicidal thoughts, as well.
Eventually, we ended up having bloodwork done for his allergies (he's got a lot of pollens/molds that are released throughout the year... so symptomatic all year long except for about 2 weeks in December), and started on immuno-therapy (allergy shots). He has been getting them for over 2 years now, and it is a HUGE improvement. He still takes a daily OTC allergy med (zyrtec or allegra), but now he only uses 3 tissues a day, and some days NONE. Instead of filling up the floorboard in the car with used tissues just on the ride to school.
If it were me, I would not give the singulair. Certainly not until I had at LEAST talked to the prescribing doctor about the side effects listed--- at length. I can't think of any reason that your son wouldn't be fine to not take them (but I'm no doctor, of course).
ETA: from their own website: http://www.singulair.com/montelukast_sodium/consumer/asth...