Hey! I'm a current MD/PhD student and I am very sorry this is happening to you.
I agree that not having a PI LoR is a big red flag for admissions, so I think your best course of action is really to at least try to repair the relationship with your PI, even if it involves changing your plans. You spent a considerable amount of time in his lab, published and he is a big wig. You really want his letter. The problem is, seeing that he is an ass (not uncommon trait among talented researchers btw), that I wouldn't be comfortable with what he writes even if decides to go through with it. You don't just want a letter, you want a great letter, and if he's behaving like that I don't think you can be confident he will write it.
If this doesn't work, I guess a letter from a senior postdoc coupled with someone in an allied lab (someone you maybe have joint lab meetings with?..) who knows your work fairly well is the best you can do, plus write something in the MD/PhD statement about this situation, but yeah, I agree with others that it must be a carefully written story.
On a more personal note — something similar happened to me when I was getting my Master's, I did not get a letter from a PI I worked with for 3 years. He was an ass and your PI sounds very similar to him. I didn't get his letter for Master's (I asked a postdoc to write me one) but I came back after 2 years, explained my situation and helped him with some administrative troubles, and he agreed to write me a letter. I don't know what he wrote, but hey, I am in an MSTP program.
Again, very sorry this is happening to you. This is unfair, but so is life.