Tufchoices,
There were two young men at my H's rehab. I met their parents during family program. These parents were very successful people who deeply loved their sons. They were so weary as it was their third time thru an inpatient rehab. This was my first, and I was very naive. One father started crying right at introductions. I had no idea how heart wrenching family program was. At the first break we could see our qualifier and their sons were handsome and affable. They were clean, which they had chosen. My H explained to me how fluid the rehab was and anyone could get anything if they wanted. I was shocked by this too.
So this rehab they claimed was it for them. They told their sons they loved them dearly, but they had to let them go. They had done all they could. I don't know what happened. But they haunt me. I pray my young son does not have his father's addictive personality DNA. You are definitely not alone and may be able to find other parents to bolster you at Al Anon.
You stick to your boundary. It is for you. It is for the rest of your family. You did really well even if your heart is sick and you are questioning the whole thing.
If you are a faithful person, entrust your son to your God. If you are not a faithful person, you might need to work through your sense of a Higher Power. Letting go with faith does help the heart sickness.
Does he have a key to your home or garage access? You might want to do a bit of safety prevention since you have younger kids. I know he flew away, but he could bounce back. You might want to coach the kids if older brother shows up, do not let him in. Get an adult.
Peace and resolve. Welcome to SR. There is a lot of wisdom here in various threads and stickies.