Hi fellow Aussie!
I'm probably the least qualified re JWs, but I did raise two kids as a single working mum. It seems to me that you've already started "deprogramming", and despite temporary hormone interruptions, that genie won't go back into the bottle.
Of course it's possible that he and his girl will become "laid-back", moderate jw's. But what i'm reading here is that the WT is increasing control. also a lot would depend on the girl and how much her family can influence her (assuming they are all "in").
Don't know the circumstances, but I doubt that Satanus' idea could be made to work. I was a very headstrong teenager once!
Most of all I'd urge you to stay close to your son. If possible, arrange outings and interests that he would miss as a "good jw". Keep him thinking and respect his opinions. If at all possible meet the girl and do the same.
Finally make sure that he knows you are always there for him, so that whatever happens, he has support outside the WT.
Best of luck, Retro