There are cabs, buses, all kinds of ways for him to get to rehab. He can call a friend.
It isn't that unusual to get cold feet.
If you want to file for divorce, I suggest you do it. If you aren't ready to, don't. But I strongly suggest you not make a decision contingent on whether he goes to rehab on the date YOU would like for him to go. And most important, don't make threats you aren't prepared to carry out.
Here's the deal. Even if he goes to rehab, he may not stay. Even if he stays, he may relapse. He may relapse many times. He may get sober this time or after a few more months of being miserable, or he may never do it. You can't possibly know, or even tell by how he "seems," what will happen. He may with all his heart believe he is going on Monday, and everything can still go south. That is the nature of the beast (and I'm not talking about him, I am talking about alcoholism).
My own suggestion is that you let him do what he will, and you get busy and work on your own recovery. Have you been to Al-Anon?