I went through this with my husband last year. I call it a relapse because since college the times its happened have been infrequent. Last year was bad. This was basically my thinking:
Relapse happens, and its not failure. There is much talk in the medical community to start calling the problem a substance use disorder.. and understanding that it can be a relapsing problem does do just what you said.. 1. You know it can happen 2. you can prepare for it emotionally and physically/financially (the last part I hadnt really done.lesson learned).
The biggest thing that helped me was learning to stay calm, respond calmly, think rationally. Remember many negative behaviors are symptoms of this awful problem.
I told my husband I wasnt upset that he relapsed, but that he needed professional help if he couldnt stop on his own. There is a lot of shame in dealing with this stuff, so I tried to take all of that out of the equation. Its a medical problem (which he also believed prior to this).
And at times he would open up and talk about what he was feeling, which seemed to lead to how his emotions got out of control, he was triggered, and ultimately the relapse happened. So we also talked of the underlying issues at play, and not just the addiction issues.
It wasnt like husband just jumped at the chance to make changes, and he struggled to find the proper source of help. Eventually he got a psychologist who specialized in addiction medicine and started therapy. The thing is professionals often know how to motivate and work with a person. There is a whole process of change model involved.
I dont think the major issue is that we as family are too close or arent knowledgeable enough to offer guidance or support.. its that often we cant handle our own emotions and can flame up whats already an unstable situation. And we also make ourselves miserable in the process.
There are behaviors just like what he told you - if you came down on him then it would be his excuse to use. Anything can be an excuse to use, but we often play right into it by not stepping back when we see this dynamic happening. Wait, step back, and that mood of his will change.
I found that when he was high or drunk then I couldnt talk to him, but when he wasnt then there were opportunities. Also at these times, I would also talk about me. I tried not to guilt or blame him. But there is no reason to cut someone out of your life at times when they are semi stable.
My husband was drinking and using drugs. There was a couple times I had to move out because he was actually physically violent when high, but it doesnt look like you have this problem to deal with.
Hope this helps a little. I like that you are trying to prepare. He's lucky to count you as one of his friends and supporters.