Brisbane area, Spring Mountain about 35 miles SW of Bris airport.
So I'm assuming "Greenbank"... if you put "Greenbank Queensland" in the text entry box located at:
Gliding Australia
You'll see all the GFA clubs and approx. distances from your son's place.
Unfortunately, most club based gliding clubs in Queensland only fly on weekends or public holidays. To get clubs running 7days you have to travel to NSW or VIC.
Closest seems to be Boonah Gliding Club (46km away) - which at one point use to fly during the week via prior appointment in a K21 self-launcher.
G.Dale coaches at Kingaroy Soaring Club (170km away) from time to time...
I did most of my flying at Darling Downs Soaring Club (151km away) - as it was the closest to my parents place.
Lake Keepit Soaring Club (8hrs drive away) is now managed by an ex-Lasham staff member and from all accounts comes highly recommended if you want to make a week of it.