Welcome to SR. You have found a great place, with lots of Experience, Strength and Hope (ES&H) and folks that are willing to share that ES&H!
I don't know what to do.
Stop doing the DANCE. I know it isn't easy, far from it, but he is and has manipulated you to the point that now when he says he's going to quit, you RUN to the rescue. Instead, RUN the other way, to Alanon and/or Naranon meetings.
I too wonder what this is teaching your son. Our children learn a lot by our examples.
This man has you to fall back on. You say you buy his groceries, WHY????? If he is using all his money for pot, oh well, his problem, NOT YOURS.
There is a way out of being Co-dependent, honest. It can be hard, but at the end of the tunnel, life is so much better.
You "love this man." Nope, you love what you think this man could be. You obviously don't love the pot or how he becomes, so in essence, you love THE DREAM not him.
Sorry if I sound blunt, I am not trying to be mean, just stating what I see from what you have posted.
You NEED to take care of YOU and YOUR SON, not this man. Please, please try Alanon or Naranon or both. Many times there are more Alanon meetings available than Naranon so it is easier to get to one than the other.
You will learn how to set boundaries and stick to those boundaries. You will learn the 3 C's:
You didn't CAUSE it,
You can't CONTROL it, and
You can't CURE it.
Also, please keep posting and let us know how YOU are doing, we do care very much!
Love and hugs,