Checked him this morning and nothing was new... hadn't spread, but hadn't gotten better.
I checked again at noon -- almost back to normal! He has a slight red mark at the bottom of his withers, and he's still a wee bit sore, but otherwise he's just fine.
Sorry, My2Geldings, I would've gotten pictures if, one, I'd thought about it, and two, if it was still there!
I washed his back an hour ago, too, so I assume that by tomorrow or Saturday he'll be ready to go.
Thanks to everyone!
Maggie (and Cooper!)