For the most part, freight trains just kind of run when they run. Some of CP's priority trains are "scheduled" but they aren't precise enough that you'd be guaranteed to see the train go by at an exact time on any given day. The tracks that run through Edworthy Park are the transcontinental main line though, so stick around for a few hours and you'll likely see something.
if you do have a radio scanner, references like this might help:
http://www.radioreference.com/apps/db/?aid=8447
Listening to radio chatter might seem kind of cryptic unless you understand what you are listening for, but at least it might give you an idea of which trains are where and when.
Just a little extra note that I'm sure you've already got covered, but please stress with your son that although trains might seem pretty cool they are also really big and unforgiving, and railway tracks are not an awesome place for kids to hang out. Please teach him to observe from a safe distance and always stay very aware of his surroundings anytime he is around the tracks.
Good luck with the trainspotting.