Seeing as it's happened four times something tells me you may be missing something. I'm not from Brisbane or involved in the classes, but maybe give Tony a call or email. His contact details can be found on the Contact Us page.
Also, don't presume that your son will be capable of teaching because he's been to a couple of instructor training sessions. You should also be aware that if your son steps into that role or labels himself as 'instructor' within his community there are responsibilites you should consider. Like, what happens when whilst 'instructing' a kid, he falls and is hospitalised due to a serious injury of some kind. Parents may want to point fingers, and you know who they'll be pointing them at.
Maybe it is worth trying to organise Tony and/or other APA instructors to come to you guys to give some good quality instruction to those that need it. Again, maybe worth getting in touch with him.