Cabeza had this EXACT same thing! It turns out that he had gotten something in there (we still don't know what) and then it hurt, so he licked and bit at it until it got infected. We ended up putting him on antibiotics and also a topical antibiotic ointment. We had to squirt it between his toes for a few weeks. It eventually healed up.
Before we had him to the vet, though, we were putting a sock on him to keep him from messing with it. What we had to do was take one of Mark's big ol' sweatsocks, put it on,and then take another one of the socks and cut the elastic off of it. We used the elastic to sort of tie the other sock onto his foot so he couldn't pull it off. Worked pretty well.
Good luck! I'd probably get him into the vet if I were you. A simple antibiotic works wonders!