Poor Marley! Thank you for providing pictures of the area. If there is pus, that means the sore is infected. He will need to see a vet for antibiotics to treat it. Tonight, wash the area with warm water and antiseptic soap to clean it, and then take him into the vet in the morning to have it examined and treated. Put an Ecollar on him tonight to keep him from scratching at it and possibly making it worse. You can make one at home, and I will put a link below on how to make one.
While at the vet, I recommend having his ear examined as well. I notice the sore is below is his ear, and I'm wondering if he has been scratching excessively at his ear causing the sore. This can be a sign of an ear infection. The vet can examine his ear, and if he does have an ear infection, the vet can diagnose the cause (ear mites, yeast, or bacteria), and can prescribe treatment for it. Good luck, I hope this helps and thank you for contacting Pet Coach!
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