Hey, my friend, so glad you're son is back safe and sound. My friend is going through the exact same thing with her son. She is letting him take responsiblity for his actions. (He got arrested with some friends for smoking pot). Seems to be working.
I am glad h stepped up and did the right thing. I am sure he was very frightened for your son also. I think it was good for your son to see your h so upset.
Just be sure to keep reassuring your son that you are there for him, but ultimately the choices he makes are his responsibilty.
And dont beat yourself up for your backslides. We all have them. We are human afterall. Just get back on track and try not to do or say anything that will seem like pursuing or pressure.