Hello there. My name isXXXXX and I am sorry to hear about your kitty. The sore on its tail is most likely spreading because he is licking it. Licking it causes more irritation and will make it larger. I would definitely have him seen by your vet to get him examined and then have the wound cleaned and get some antibiotics for him. He also will need an e-collar (cone of shame) or else he will not stop licking it and it will never heal. Also he will most likely need a pain medication injection to help reduce any pain or inflammation. Until you have him seen just keep the wound clean with warm soap water if possible. You do not need to apply any medications at he will most likely lick them off until he wears an e-collar. I would get the carrier out tonight and let it sit out for a few days until he comes out. Otherwise catch him when you can place him in a small room with food water and litter box until your appointment then put him in the carrier. But again if he isn't treated it will not heal and will only get larger and cause more problems unfortunately.
I hope this helps. Please let me know if you have any additional questions or concerns. I would be happy to help. You can purchase e-collars from pet stores so worst case scenario if you cannot get him to the vet you can buy an e-collar and place it one him so he cannot lick the wound, keep the wound clean and apply triple antibiotic ointment to the wound and watch very closely. But if it worsens at all or doesn't heal he has to be seen. Please do not forget to rate my response so I can receive credit for my time. Thank you!