There are lots of things that can be done to help, but all that I know require some level of cooperation/willingness on his part. Also, some effective things can be costly, and although some third-party funding (foundation, government) may be available, it is not so easy to get, so don't count on it until you have it.
If he had been seeing a (good for him) therapist and is willing to go back, that would be the first step. If he hasn't seen a therapist, I'd suggest trying to find one and persuading him to visit. Also look into (anti-)drug programs in your area. Some of them offer things beyond just "don't use them", such as weekend outings, including camping trips.
I'd expect he is sufficiently addicted that some kind of de-tox would be in order, perhaps at a good psychiatric hospital. That would be followed by whatever program you can arrange. I know of one young man who was in similar circumstances at the same age and who did ask his parents for help. They gave him some limited choices, including a psych hospital (where he stayed some 10 days or so), followed by a particular kind of wilderness program (they are not all the same). It truly helped him turn his life around. Things are not all peaches and cream now, but he has a reasonable future and real responsibility for self. Hospitals can cost over $1,000/day, I believe, and wilderness can easily run over $20,000 if all done on a "private pay" basis. However, there are some easier-to-arrange funding sources for the hospital portion, and as noted, some aid may be available for the rest.
I couldn't just do nothing and hope for the best if I were in your position. But you can't do it for him either. So, check with local social service agencies, including any local drug programs, for any suggestions on available resources and programs. And good luck to you and your son. He can achieve real growth and success if he makes the commitment to it.