This may sound harsh, but by providing a safe environment with no responsibility, your parents are basically enabling him to continue with his lifestyle- which in the long run is not doing him (or them) any favours.
As others have said, he needs to hit rock bottom, and decide for himself that enough is enough. Let him contact the local authority for emergency housing if he needs somewhere to stay.
I would suggest kicking him out of your parents house, and letting him fend for himself. Distance yourselves from him.
Rehab at this stage, when he always has a safety net, would just be a waste of time and money.
Tough love.