Someone at the barn said it was edema- just an accumulation of fluids and it would go away on its own, but it's usually from some kind of trauma like getting kicked. He hasn't been out with other horses since Saturday (Sunday we had an off-site lesson and were gone most of the day, and he's been on stall rest since then) Is it possible it's from him standing awkwardly due to the injured leg, even though it's a rear leg that's injured and the swelling is on his chest?
My vet boards at my barn and is there riding pretty much every night, so normally I'd just ask her, but she's out of town for a few days... There's another vet at that practice who I've never actually met, but I can call if needed.
Here are a couple pictures: the first one is from Tuesday, and the second one is last night.