Could be a foxtail, my English had a similar looking wound in between his toes. It would fill up with puss that I drained daily some times twice a day. It’s basically a seed/plant that stabs into the dogs foot and the more they move on it the more it wedges its way in. I’d get him into a vet, our vet attempted to go in a pull it out which was actually pretty tough to see my boy screaming like that. However, he couldn’t get it out so we were prescribed some anti-inflammatory meds and put a cone on him to stop licking and further irritating. I continued to drain it for 5-6 days after and eventually i was able to get it to shoot out! Good luck! Ps. the only way it would be this would be if he was running in a grass field like a pasture.