Erika, I find my chickens respond very well to herbal remedies, I keep a number of herbs on hand. You can make a strong tea of golden seal, echinacea, chapparal, or other anti-infective herbs, and add to his water. That way if he does have any infection, you can begin treating it from the inside out. Bag balm is amazing stuff, too, I've used it on many a wounded bird, with great success. They get it all over and look truly awful for a time, but it works great.
We've had rouge foxes here, twice. Don't give the fox any warning by running and shouting. Just keep the gun handy, examine your situation, and envision different ways you might be able to give yourself time to aim and shoot, before the fox knows you see him. Even though the air gun probably won't kill him, it's possible that a good shot will deter his return. I'm been told that if they get a non-fatal hit, they won't be back, but I don't know if that's true. It could be a myth. But it'll sure make him think twice, and if you shoot him more than once, I'd expect he'd either die or give up, eventually.
My DH managed to shoot the first fox, he kept running, but never came back. I assumed the shot was fatal, just not instant.
The 2nd fox, a couple of years later, my funny-looking little brown dog Cleo caught and chewed on, but she isn't big enough to take down a fox. That one kept coming back, she kept catching him, and kept him from getting any chicken dinners. She literally rescued hens right out of his mouth. One was so badly wounded that we went ahead and butchered and ate it, (it was a meat bird anyway)
but the fox got a hard run, chewed on by my dog, and nothing to eat for the trouble. Same thing 3 more times, then he tried my neighbor's down the road, who was able to get a good shot in and killed him. We knew it was the same fox because his back foot was chewed up, (classic Cleo damage).