Katie,
I'm so sorry about what is happening with your son. I agree, NO MISTAKES. You made a decision to let him come back based on what you knew at the time. We all keep learning as we go along. Unfortunately, there is not an exact rule book on what to do.
Sticking to your boundaries is key though - as much as it hurts. My husband used for 20+ years - mainly because he kept having a soft place to land with his family. It wasn't until he knew that they really were done giving him places to live and chance after chance that he finally got sober. I think that he would have found his bottom a lot sooner if he had had to fend for himself.
My oldest son has gotten into pot smoking and I've seen how different it feels to deal with a child that is abusing drugs vs. a mate. It wrenches at my heart in a whole different way.
We're lucky that there are so many wonderful moms on our forum....they really know the deal and certainly have a lot more insight than I do. I am thinking about you though and sending healing love and light.