i could be wrong and hard to tell from the pic but that looks to me like an allergic reaction yeast. Looks like some yellow on the pad as well. If that is the case then it could be from the food or just summertime in general. Sarah has what looks like that on her paws. Only the front one is the worst the others are just kinda red and one is not really red at all. Im thinking a interdigital cyst will form at some point also. You can give him a 1/2 a benadryl and see if it gets better. let us know what the vet says please and I hope all goes well.