I would say the best way to help him is to get him to a doctor ASAP. From there he will hopefully end up with the proper treatment for detoxing and possibly a treatment center for his addiction also. A treatment center will create a complete change in his normal habitat along with a break from the people he hangs with. It will also create enough clean time before he gets back out into the world for him to get his thinking turned around. If nothing changes nothing changes. It's as simple as that. He needs to change his playground, playmates and playthings. Most importantly he's got to want to be clean, or he'll just bide his time before he goes back out sooner or later. Like Kyle said above, this won't be easy, especially without your son's cooperation. I just went through this with my daughter for over 3 months and I felt completely powerless. Now she's doing well...one day at a time.
I agree getting him some professional help for his addiction. In the meantime you can look up the Thomas Recipe at the link I will provide to help him deal with his withdrawals.
http://www.medhelp.org/health_pages/Addiction/Thomas-Recipe-Re-Posted/show/16?cid=66
Fentanyl is a very strong narcotic pain med....second only to Dilaudid. Take him to the doctors if you can.
Best to you and your son.
You can help him by getting him some professional help so that he doesn't continue experimenting with addicting drugs he is so young to be doing drugs. Hopefully it's something that can be over come with the proper help, wishing you the best and a drug free life for your son.
More info on his use, where he is getting it etc. would be helpful. Fentanyl is very powerful. From what I have read, it is stronger than morphine. I have used it once and it was very strong. I can see how it could be very addictive.
Do you know anything about the patch? How long was he using, etc.?
And how did he get the med? And, why do you think that he'd be able to go out and get something worse? Why did he start using in the first place?
Detoxing and staying off fentanyl can be very difficult, especially at his age, and especially if the only reason he's quitting is if he can no longer get the med, if he was caught, etc.
If you can shed some more light on his situation maybe we can offers some suggestions, but at 17 you feel invincible (even if he feels like crap right now), so this is going to be difficult.
Hang in there.
K