Sherril...I'm so sorry. I am just thinking and feeling along with you about your son.
My son is now 26, but there were many many times when I wondered exactly the same things you wrote. Why in the world? In my son's case, he was addicted to prescription drugs and continued to get into all kinds of legal trouble associated with drugs. A to Z. Also, he can't drink alcohol and that was one of his gateways to the Rx. He also smoked pot.
The downhill steep slide started (to my knowledge) at about age 19. First semester of college. Flunked out. My son had it all too, youngest of two, with every privilege you can think of, also with requirements and consequences, all along. He was privileged but we expected them to do their part, and that didn't mean just sitting around playing video games all day long.
One thing my son said that still echoes in my head: I don't want to grow up. He started saying that as a little boy and continued saying that into high school.
I think he has been afraid of responsibility, adulthood. He is anxious and has had some depression. He is a very hard worker and smart (both book smart and common sense smart) but I think he doesn't feel like he is good enough or fits in. I think substances helped him feel better about himself and the world around him. At the same time he was grandiose---he would say he was going to be rich and live in a mansion.
All classic addiction behaviors and thinking.
I don't think there is anything, short of locking him in a cage, that could have stopped what he decided to do. He had to go the whole distance. He wouldn't listen to anybody.
So, to your question---how to help but not enable. Your son is now 18 years old, an adult in our world legally, but you probably realize, like I did, that chronological age and really being mature were two very different things.
My son was just a kid at 18, and in many ways still is. But that doesn't mean he's off the hook, from an adult standpoint. Society expects a lot from people over 18 and the consequences of not delivering are harsh.
I don't think we can do it for them. We can't talk enough, reason enough, sit on them enough...to get them mature. I think they have to experience life on life's terms, and again, that is some harsh stuff, sometimes, IF THEY CHOOSE to go down that road. Your son appears to have chosen the road, for now.
I would suggest just loving, encouraging and supporting him. Not with empty platitudes, "you're great" but with real encouragement when he begins to show more mature behavior and thinking, i.e., "I like what you just said. That makes good sense." That type of encouragement.
Tell him you love him.
If he says he might try an AA meeting in jail, say: "I think you might learn some good things there."
If he wants books, I would send those. I sent books at the beginning, the first handful of times he was in jail, but finally I stopped.
I would put some money on his account, I believe. Not a whole lot, but a little bit from time to time for essentials.
With my son, I put money on the account for a long time, but then I learned he was using it for winning and losing while playing cards in jail, so I stopped.
I wrote letters and postcards, and over time, I learned not to preach about what he needed to do in those letters. I just wrote what I was doing and what was going on in my life. I always said I love you and I'm hoping for the best for you.
Your son will learn a lot about life and about himself where he is. If he can gain a good perspective, he can learn a lot of valuable lessons.
I got the point where I was actually glad my son was in jail when he would go back to jail (was in 8 or 9 times). At least he wasn't homeless. At least he had 3 meals and a place to sleep. At least he wasn't using drugs (theoretically). At least...
I could relax when he was there. It was better for me than the alternative. Although at first I thought I would die the first time he went there. It felt awful to me.
Hang in there Sherril. We can't know his journey. His journey is his alone, and IF HE CHOOSES, he can turn it all around and have a great life.
We're here for you. We know how hard this is. Try to use this time to focus on YOU. Warm hugs.