He may have stepped on something or if he was alone outside something may have bitten him. You can try soaking the foot in epsom salt water, this should help him feel better and will take away some of the swelling. Putting something topical on it like neosporin probably won't help since he'll just lick it off. He may have something stuck in the foot so if it doesn't resolve itself within the next couple of days then you should take him to your vet to see if they can find anything in the foot that may be causing the continues swelling and irritation. Hope he gets better soon.
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