Magic, I am a bit late to this thread but as soon as I read it, I wanted to post.
My BFF was in your position. She met her DH when they were both in their early 20s and both weed smokers. Over the years, she stopped smoking, but he never did. By the time she hit 40, his tolerance was so high, he was daily smoking skunk that was so strong it would give normal smokers something similar to a psychotic break.
After nearly 20 years, she kicked him out ... and it was only then that she realised how devastating his habit had been.
He had spent upwards of £6,000 a year on drugs for nearly 20 years.
As pp have said, he would never leave the country on holiday because he couldn't take his "weed" with him.
He stopped socialising or wanting to go anywhere because it would be an evening where he couldn't smoke weed.
She used to end up trapped in the kitchen in the evening because the sitting room was impossible to sit in due to the sheer level of psychoactive smoke in the air.
There were massive issues with his work etc.
But ... the thing that she realised that really shocked her was that he couldn't remember anything about their lives from one day to the next. Those twenty years were, more or less, just a haze for him because he was stoned every single evening.
She used to get very confused because they would have a conversation and agree on something, or something would happen, and the next time she brought it up, he would declare it had never occurred. She started to think he was gaslighting her, but the reality was ... as he was always stoned when she spoke to him or they did something, he couldn't never then remember it at a later point.
He couldn't remember conversations, plans, events, agreements, special memories, where he had left anything ... he was permanently in the moment with scant recollection of the near, medium or distant past and no grasp of the future whatsoever. And you just can't live with someone like that; it's like living with someone with self-induced anterograde and retrograde amnesia.
And the reason why I wanted to mention this to you is that ... well ... how much of your DC's childhood so far can your DH actually remember? If you sat him down and asked him how much he remembers of your DC's fifth birthday, would he remember anything at all? What about what happened last weekend? If he is constantly smoking, how much of DC's childhood is he actually mentally present for?
The thing is ... if he is pretty much constantly stoned, he 'aint present. He might as well be a ghost trapped in some different existential dimension. How can this be a "relationship" when he probably can't remember the majority of what you say to him from day to day?