Jodie, I think you might consider writing how your son's actions impacted you, for YOU. Send it to him, but remove any expectation you have about what the outcome might be, write it for
you to be clear about how his actions have impacted you and so you can then let it go. Sometimes those kinds of letters can be cathartic and help us to accept what is and let go.
It's kind of like the 4th step in the 12 step movement, where you make amends, where you do a "searching and fearless moral inventory of yourself." Whether your son chooses to hear you or not is up to him......but the putting it together part would be for you. It's usually for the addict to do the 4th step, however, I've done similar kinds of things in therapy which were for ME to put it all down and for ME to heal from it. Do it for YOU Jodie, not for your son. You can't control what he will do with it, but in the meantime, it may serve to help you to let go.
In the letter you could map out what you are willing to do and all that you are not willing to do, starting to set some real boundaries and this would serve as his notice of your intent as well.
Here is some info on the 4th step with some guidelines, perhaps it will give you a place to start......
https://addictionrecovery.lds.org/steps/4?lang=eng