Is the leg in a normal position, no abnormal angles or loose floppy appearance? The best way to tell is to gently hold him and examine his leg. You don't need to do much, just gently pull on his toes and see if he can bring the leg back up (resistance to you) or tap it from the side and see if it seems loose, kind of hinged in the wrong direction. A leg bone should not swing side to side.
If he can pull his leg away from you and the bones seem stable, he'll probably be OK. Just has some soft tissue injury. But if he has any of the problems above, he should be seen by a vet ASAP. If he doesn't start bearing weight on the foot in a few days, he needs to be seen by a vet. Only an x-ray will tell you for sure if there's a break or luxation. It's up to you. But try to imagine how you would feel if your leg as broken. :-(
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