Sometimes bites on a cat are very hard to locate, because a cat's skin can heal over them very fast and they aren't visually discernible, even though infected. You might not see one until it begins to abscess. Is he exposed to other cats with whom he might fight? Does he hiss at you if you touch him anywhere on his hip or leg? It does sound like a bite to me, and if so, the trick is to locate it and lance the abscess. Antibiotics, not anti-inflammatories, would be called for after cleaning out the wound.