Putting the legalities aside for a moment- there's no way your child is not going to find out about this if you pursue it. His employer will notify him of the request, as will his bank. Unless he has entirely disengaged from education and training and is working full time, it is extremely unlikely he will be earning over the 350/wk threshold which would reduce your payments.
How many more months until he turns 18? Let's say super optimistically he earns $400/wk - which would be very generous for an under 18 in casual employment, usually it's more in the 250/wk ballpark and goes up to just over 300 if there's public holidays in a pay period.Your payments would only reduce by around $20-30/month as a result, if that. Even if it's a full 11 months until he turns 18, you'll have saved aournd $300.Chances are you'll save nothing, nada zilch, especially if he's at school, TAFE or uni and not working a 40 hour week...
What will you have acheived for this whopping amount of money? Advertising to your child that you're not afraid to interfere in their employment or private finances out of spite, that you resent him and see him as a burden. That may not be accurate but teen brains take these things personally - he's not going to see it as you resenting your ex, he's going to see it as you going to war with him personally and interfering with his growing independence.. Anything CSA can do will likely make things socially awkward for him at an age and stage where he's acutely sensitive to embarassment. You're shoving him into the middle of your resentment of your ex and making him front and centre of whatever battles are ongoing between you.
It sounds like you currently have contact with your son. In a few short months there will be nothing you can do to force him into an ongoing relationship through the Family Court. He will get to choose if he remains in contact with you from then on. Why would you pursue this madness which completely risks that ongoing relationship by advertising you resent him, see him as a financial burden and are willing to advertise that you see him that way via his employer or bank.? Its one hell of a spanner to throw in the works for a few hundred bucks at a time when his legal obligation to have a relationship with you is coming to an end.
You've been able to manage the payments so far and they will stop in a few short months. Why on earth would you gamble with your relationship with your child just to save what is likely to be a double digit amount each month? I know sometimes CSA can seem unfair and to pursue these things is a matter of principle.It will be over soon enough. The question is, is the principle of the matter worth risking alienating your son?