Just chiming in as my ex was a heavy cannabis user. His relationship with it went up and down over the years, and he really battled with it. I knew quite a few other men who were exactly the same as him.
You will get people saying it's harmless. Just like any drug, for some it is. For some it's helpful for their medical conditions. I am not in the camp to say no one should have it, but if you are living with a constant smoker who has a problem with it, it really isn't helpful for recreational users to shut down someone's concerns.
I dealt with terrible moodswings, serious panic as he was running out, making me pester friends if he was having trouble finding more and awful moods if he ran out or tried to give up. He tried to get me to smoke with him, I guess to make him feel better about it. I did for a little while and stopped. Now I can't bear the smell, especially kids smoking skunk when I'm in town. It's horrible.
I once worked up the courage to tell him very honestly how it was affecting me, and him, and asked him to stop. He didn't. He said it helped him sleep and it "stopped" his brain (he had anger management issues and mood problems.) After all that time, it was hard to know which were linked to cannabis and which were part of him, from before the time he started smoking.
He said the same as your partner - it helped him cope.
It was linked to 10 years of EA and as things got worse, I wouldn't stand in his way to get some to stop the bad moods and have a "happy" OH again.
You aren't being dramatic, at all. Not going on holiday because he can't access it is a problem. We went on holiday to Amsterdam several times, and he went with his friends too. Guess why?