If it's legal in your state, I would expect there would be some sort of medical board you could report him to. Either way, this might be a sign that you need to find a new GP, if they are explicitly stating that they are going to refuse treatment over a drug test showing something you have a script for.
Small question: Did you remind him he had told you it was fine previously and that you had a script for it? It might help to state the question very directly: "I just need to make sure we're on the same page here. You are refusing moving forward with the next steps of care due to a drug test showing positive for a substance I have a perscription for?"
You can't assume they'll remember this shit or be aware of your other scripts. A new person took over for my retiring GP and somehow got the bright idea to refill me at double my fucking dosage. Supposedly it was recorded that way in my file (despite never being a problem for literal years).
They're just calling this new thing BMI2 as shorthand