Hi mkr. Why would you not believe he will always be an addict when there is no behaviour to rule this out. He is using, he's not in any kind of recovery?
I have no idea what your life was like before you met him, but does this sound even remotely "normal" to you:
I have been justifying him recreationally smoking heroin in my house for months.
Smoking heroin in your home. hmm
His mother literally thanks me for not kicking him out and giving him a “safe” place, so to speak
So he has a history of being enabled then, that's not surprising.
He orders it from the dark web, he says it’s better because unlike the street heroin around here it isn’t cut with fentanyl.
This is just addict-thinking. He has no idea who is behind the drugs, surely the "street heroin" dealers aren't on the dark web! He wants to believe that but he has absolutely nothing to back this up.
I do believe you know all this already. Certainly you have to accept that for now your boyfriend is a heroin addict, or don't accept it. If you can't you need to move on. You can't change him.
The question is, what do you want? He may be worthy of love, however are you his caretaker or his SO? What is in any of this for you? Is this a good place for you? Are you happy?