It sounds like you are willing to do some soul searching and some personal recovery which is AWESOME! It's a good starting point - a much better starting point than "how do I stop my boyfriend from using again"? (The answer to that question is you can't.)
I strongly recommend melodie beattie's book " codependent no more" and also nar-anon or al-anon. Work the recovery you wish your boyfriend would work. And then, no matter what he chooses, or what happens to your relationship, you have grown and become a stronger, more fulfilled and self-sufficient person.