It's hard to say since we can't see it. The most common cause is a foreign object in the pad or a trauma to the leg. It could even be a bug bite. I would go ahead and see the vet with him if you can't find any evidence of trauma or a bite mark.
Cats cannot tolerate OTC medications so there is nothing that you can do for him at home, aside from massaging the foot to increase blood flow.
Let me know if you still have questions.