Help! His pastern's swollen!

Hi!
My 11 year old TB came in from turnout 2 weeks ago with a HUGE swollen hind leg. I cold hosed it, which made it go down a bit, but his ankle, his second pastern, is still swollen and feels like it's full of fluid. Ts warm, yet he's not tender.
He's sound, but im worried!

Can anyone help?

I would call the vet. Could he have strained it somehow? Until the vet could come, hose it with cold water, wrap it and hand walk him to keep the swelling down.