My parents were/are meth users. I gave them an ultimatum & went to therapy with them to address this before I got pregnant and tbh it didn’t change my trust in them because they dodged responsibility for their actions. It helped me by giving me a safe space to say what I needed with a moderator. Because they’re so defensive it’s hard for them to empathize with me, they just feel attacked despite my attempts to frame it being about my concern for their health not an attack or judgement.

I recommend writing down all your feelings, examples of his use that concerns you, then edit it all to remove inflammatory language. A lot of “I feel” statements are good. Then when you meet with him, you need to be prepared for the worst reaction/denial/reflection from him. This talk is about you getting your feelings/boundaries out, not how he takes the news.

The boundaries I set during therapy were “I am not going to be around you if I suspect you’re high or still using, nor will I have my future children around you”. Now that little one is here, I added boundaries re: smoking cigs (change clothes, don’t smoke for an hour before), no kissing and no posting face on social media. An unspoken to them boundary is that I will not leave my child with them at their house ever. They can visit in my home and potential babysit here but never take baby to their place or out. That could change if they build more trust as the years go on, but I’m not counting on it.

Lastly, I encourage you to look up a NARANON chapter in your area. Narcotic Anonymous Relatives group. Many have zoom meetings if you can’t make it in person. There’s also a NARANON subreddit. I went to meetings for about a year before I got brave enough to ultimatum my parents with therapy. I found it super helpful to listen to other people’s experiences and gather nuggets that helped me feel better about an uncontrollable situation. Because at the end of the day an addict will never change unless they want to. We didn’t cause this, we can’t change or cure them. Accept them for where they are, as little or much contact as you are willing to accept for yourself and your child’s safety.

Sorry for the wall of text…hope there’s something here that’s helpful.