Theophylline and Terbutaline (T & T) are inexpensive and have been recommended for SCLS for decades, so they are the medications of first resort, despite their nasty side effects. However, T & T don't work for everybody.
Singulair is completely unproven as treatment for SCLS but is often given nowadays because it is a similarly inexpensive alternative to T & T and it doesn't have many side effects.
IVIG is hugely expensive, there is less literature on it (all coming from outside the USA -- although I'm working on fixing that!), and insurance companies here usually deny coverage and one must appeal and fight them. ("Been there, done that!") Thus, it is the medication of last resort in all countries. (You live in Canada where medicine is socialized, so your doctors must be persuaded in order to prescribe IVIG, and they of course are keenly aware of the cost factor.)
If it's any consolation, children have been known to outgrow their SCLS problems, although we have several parents of SCLS children (mostly around your son's age) in this community and none of them has reported that their children have been cured just yet. (Perhaps they have been cured, and so they stopped checking in with the rest of us and have thus not reported on their success.)
If the diagnosis of SCLS is confirmed, you might like to start a new discussion topic entitled "SCLS in children." That might elicit some of those parents to give you feedback.