Can you get together with a counselor, your son, and perhaps the boy, to decide on how all of you can best help him?
Will your ins pay for a psychological treatment facility, since he has PTSD. Would this even work, with the drug problems?
Maybe you could write down all the kinds of emotional support you CAN give him, and get his father to do the same.
My nephew died at 10 of a Valium overdose. For the rest of my life, his unnecessary death will make me sad.
While I cannot offer much advise, I understand the frustration and difficulties you and your family face dealing with a loved one with drug/mental problems. We've been there, done that. Your grandson is now considered an adult so there is little you can do to curtail his behaviors, not that much was effective when you had some control over him anyway. And really, if someone is on the path of addiction and criminal behaviors there is little you can do to influence him anyway. Of course, you can't just "throw him away" but allowing him back into your home would probably not benefit him and would drive you nuts and possibly pose a dangerous situation for your household. If you suspect that he wants to hook up with his old girlfriend, there is nothing you can do about that except not allow him to do it under your roof. People of this age and situation can be very resourceful when pursuing their wants.
Perhaps there is a group-home setting that would suit his housing needs. His probation officer might be able to suggest where to find some type of help for him. Too bad the courts have not ordered him into a long term treatment center. Some states will pay for this as it cost less than jail or prison. Get a referral from social services or the county for a counselor who can help you sort this out.
You might consider joining a support group for yourself such as AA or NA to keep your sanity. They can offer you insight, support and suggestions. Sometimes you have to "let go and let God" as we all have our free will. You are right. His father should take responsibility for him. For one his is younger than you to handle the stress, 2nd he was part of the problem by abandoning him, 3rd - he has some insight on addictions and how to overcome them. And finally it is his responsibility that he shunned his son years ago but it is STILL his responsibility and it's time for him to own up to it. Good luck to you. I know it is heartbreaking and can take you to your wits end. Send out some SOS calls and see what services are to be had in your state and county.
Great advice. So practical.