Get the referral that Monnie suggested, i.e.

If you are going to go to a private Clinical Psychologist, see if your GP is happy to refer your son under a GP mental health plan to a Medicare registered Clinical Psychologist. That way you can access up to 12 individual sessions/year that are mostly covered by a Medicare rebate. The Medicare Scheme is called "ALLIED MENTAL HEALTH MEDICARE SERVICES: PSYCHOLOGICAL THERAPY AND FOCUSSED PSYCHOLOGICAL STRATEGIES".
That way you hopefully will not have to pay quite as much.