My horse fell while lunging!!?

My 5year old Thoroughbred Gelding usually likes to buck while he lunges in the first ten minutes, which I think is okay because he's energenic and has to get the energy out someway. Well, while lunging he slid on the gr and fell down on his back left leg. He got back up immediately and I ran over to him, walking him back to his pen. Well, he puts a little weight on it and it doesn't seem to be broken and he can eat and walk on it, but we just paid for a large bill and we can't call the vet unless it's extremely serious. What can I do to help other then keeping him in his stall? :( I'm really worried about it., but we think he hurt either his thigh or his ankle, but his ankle seems okay. PLEASE Help!!!!