I am so sorry that you are going through this!
He sounds like a wonderful man who is an addict.
You need to keep yourself safe, & more importantly keep your son safe... It is one of the hardest things on Earth to set boundaries, & hold boundaries, with someone you love...
I think that - given the situation - your baseline boundaries are:
You cannot live here, because you are using.
You cannot be high when you are with me.
You cannot be high & have contact with my son.
Then, there is...but I love you & want you to be well. I miss you in my life. Do you want that or this?
It's the "do you want that or this?" which is hardest to hold. Can't have both.
People relapse. People recover. People also choose not to recover.
Keep yourself safe. Block any access to your finances. Take a deep long breath. You are in a very difficult deciding place. You have been hurt & lost trust. It's ok to say that to him. You had a dream & he was part of it. It is a loss, like death is a loss.
Then you have to repair & build your own life! Make it as just right as you can. If he is an intelligent man, he knows he has to do some serious work to join you in that life. It's a wait & see moment, but you can't just wait, you have to keep building your own life (& your son's life).
For some addicts, a relapse is a singular thing. For others, it is a coping pattern which will play out over & over.
Your only job is you & your son. His addiction is 100% his job to conquer. If he cannot, it does not reflect on you or the quality or loyalty of your love!
I know it's hard to hear, but if he cannot overcome his demons, you will live a better life without his presence. So just keep going forward, breathe, reach out to friends & family & online support & perhaps a therapist or alanon or church, & keep your life alive. Hold your line in the sand. That is the waiting. Don't soften the line!
You are in the right place! You have found a private & nurturing community to support you - no matter the outcome!
Sometimes, we just get a brief moment of good relationship! This is not a mistake. It is a glimpse of what is possible!
You cannot change or fix another human! That's their task. Life is just really complicated & hard, sometimes. Your only job is to protect yourself & your son at all costs. And sometimes, the cost is high. But, in the end, our job as parents is immense...
I send a far-away hug! My only knowing is that co-parenting with an active addict hurts all...