The cat could have just landed wrong. Almost like a person twisting their ankle. However, the more reasonable thing to assume would be that the cat was bitten. You would not be able to see the wound after the cat cleaned it. All you can really do is keep and eye on him. If he begins to limp more severely, then bring him to the vet. If you start to see blood or pus, then take him to the vet. Also, you need to be aware, that if he was bitten by a skunk or other rabid animal, he could contract that disease. He can also contract feline leukemia and other lethal diseases from other stray cats. Just be observant.
I applaud you getting him fixed and being so responsible for him. I wish there were far more people like you out there in the world today. Try to find him a real home. He doesn't have to be a purely indoor cat, but finding him a home where he can sleep indoors and be safe would be marvelous. I know how hard it is to adopt out strays, though. Thank you for loving that cat.
