Maybe he has been processing the situation and realizes it isn't the job, but his disease that is making his job (and the rest of his life) unmanageable. I hated my job, too, at the end of my drinking, and it suddenly dawned on me (yeah, we can be SLOW) that it was my alcoholism that might be making it all so overwhelming.
And, in fact, that was the case. I opted for AA (90 in 90 to start), but I also promised myself that if it didn't "take", rehab would be the next step. I never had to do it, but I know people who say rehab was the best gift they ever gave themselves.
Most places have policies (and there may be laws) that there is job protection for people going to rehab. Does he have enough sick time to cover most/all of his stay?
I don't blame you for not being "over the moon" about this. Nobody can say whether this will be the start of a brighter future, but it just might be. As with any challenge, try to keep a positive outlook while doing what you need to do for yourself. Some people really blossom in rehab. It all depends on how bad he wants it, how willing he is.
Hugs,