Hi, and welcome back. I don't think we've met. My divorce from my exah of (almost)26 years was final a couple of weeks ago. I totally relate to everything you have said about being tapped out and done with it.
I agree with others above me-don't contribute. He made the mess, let him pay for it. I doubt it will do any good at all if he isn't taking 100% personal responsibility for everything, including paying for HIS rehab.
Personally, I don't think his parents need to be paying for it, either, but that's just me.....You don't give his age, but I'm guessing he is way past legal adulthood...
Tell them exactly what you told us, hold your head high, and keep putting one foot in front of the other and putting yourself and your kids first.