Hi there bb, welcome to Bluelight. Speaking from the other end (as an addict of many drugs one of which being heroin) perhaps I can give you some advice.
While I was addicted my parents tried their hardest to put me through programs, encourage AA, they tried to cut me off from sources of drugs, and overall control my life. This had the backwards magnet effect and I just revolted against everything.
I think the best you can do to help your son is to allow him to know he can trust and confide in you, that he doesn't need to hide or run, and that you will always love him.
Get past the stigmas and the fear and talk to him, ask him about it, what it's like, what he likes about it. Perhaps ask him how long he plans on doing this. Ask him if he truly enjoys it. He will open up and a specific bond will form.
Perhaps you can see what his future goals are. I was currently enrolled in college when one night I went on a particular drug party and my parents found out. Their reaction drove me away and their anger and lack of listening and understand me drove me further and further away. I could have easily gone back to school that Monday after the weekend but they forced me to quit school (to confine me at home where I would be safe--a drug user always finds a way). Had they of told me their disappointment and accepted that I did this and trusted I still could gain footing I would not have spiraled out of control.
I wish you all the luck in the world

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