Hello and thank you for asking your question on just answer. My name isXXXXX will try to help.
Is the wound directly above his anus or off to the side?
Have you caught him licking the wound?
How are his stools?
Does he ever strain to have a bowel movement or drag his hind end across the floor?
I bet you are correct. It is an abscess or a localized pocket of infection. If it is not healing, there is a possibility that when it gets close to healing, that it starts to itch and then he licks it open again. There is also a possibility that he has a foreign body, such as splinter, stuck in the skin there, and it is never going to heal until the foreign body is removed. In addition, the bone could be infected there or he could have a bone chip there. But that is less likely in that location. So I am guessing you are trying to avoid the vet. If that is the case, he must wear an elizabethan plastic cone collar until the wound is completely healed over. You can purchase one at any larger pet store. Make sure it extends past his nose or he will still be able to reach back there. He will have a "temper tantrum" for the first few hours but then he will get used to it. He can not go outside with his e-collar on. You can also try a "bite not collar." Bitenot collar website
It will be easier to treat the wound with his e-collar or bite not collar on.
Clean the wound twice a day with the iodine. If you see a hole, flush the wound with a syringe. Try to get the iodine into the hole.
Hot packing the wound, with a warm wet wash cloth will bring the infection to the surface.
Then apply neosporin triple antibiotic cream twice a day.
The wound should be completely healed in two weeks.
If the wound is not healed, then he needs to see the vet. The wound will probably have to be explored under anesthesia to make sure there is not a foreign body in there. He will also need a oral antibiotic as well. If you want, you could just start with an oral antibiotic for a week and if there was no improvement, then spend the money on the wound exploratory.
I hope this helps.
The good news is, there is not a parasite that would cause that. There is a type of fly, that lays a larvae, that grows to about the size of you pinky nail, under the skin but it would be difficult for the fly to get under the tail long enough to lay the larvae. Then there are maggots, or house fly larvae, but you would see the maggots squirming around. If you are open to seeing the vet, that would be my first choice since I believe there may be something stuck in his skin. Unless he just keeps opening it himself. It will heal faster with oral antibiotics and an e-collar too. There is also a slight chance he has a fungal infection back there and it will not respond to antibiotics and he will need oral antifungal medication instead. But my feeling is that any non-healing wound should be checked for a reason it is being prevented from closing.