My mare did that when the fence was off. I came out that morning to find her hobbled on both back legs.
Lucky for both of us, she just stood there and waited to be rescued! No harm done except for some hair loss. Didn't even swell up.
This really makes me question the use of electric fencing. How safe is it really? I had my mare bolt through the fencing one time, it all came down and there was no damage done. Other than that she did get a couple of shocks!
Good luck! I hope he heals up and is sound for you!