Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about this poor wee one’s situation, and wanted to help.
Based on your history, I would be quite suspicious that this cat's issue likely started as a cat bite (+/- an abscess that has now ruptured) but his constant scratching and lack of medical treatment is leading to a worsening wound. In regards XXXXX XXXXX we'd ideally want this wound flushed out (using a mild antiseptic like dilute chlorohexidine or even salt water) a few times daily, covered with a light gauze to prevent infection, and this cat on a course of antibiotics +/- kitty safe pain relief (ie Metacam).
Now with the cat not belonging to your son, this is a sticky situation. Only the owner can really seek treatment for a cat that is owned. If your son knows who owns the cat, he either needs to press upon them how important having the cat treated is, ask them to give him ownership of the cat (so he can seek treatment), or get the RSPCA involved (they can require the current owners to get him treated or take the cat in so that he can get treatment). And truly, while I know it will be a hard decision for your son, it would be worth considering getting the RSPCA involved since they would ensure that cat gets treatment today but is rehomed to a household that will take care of him and provide for him over the duration of his life (the difficult but best option for the cat).
Otherwise, if your son convinces the owner to relinquish ownership, your son does have a few options to get this can treatment. First, if he is on benefits, he is likely eligible for the PDSA. (They offer this service and tend to work on a donation system rather then providing a payment system, so he can pay as much as he is able). To find the PDSA nearest to him and check eligibility, you can check HERE.
If he isn't eligible, there is still hope. Now I cannot see where in the UK he is based, but if he is local enough to London, the Blue Cross (HERE) will see animals in this type of situation and as a charity their services are subsidized significantly. London also has the Battersea Dog and Cat Home who offer low cost clinics for pets. And Londoners can also obtain low cost care for kitties via the Celia Hammond Trust (HERE).
Finally, if the above aren't options, then I would advise again contacting his local RSPCA branch (LINK ). The RSPCA does have some hospitals (ie London, Birmingham, Salford, etc) that they will see animals directly, but even if he is too far from an RSPCA hospital, then some branches can still help subsidize or even cover the costs of getting his treatment. They may even be able to help organize a payment plan and refer him to their contracted vet, who will likely have a contracted price range for their charity. As well, Cat's Protection (LINK) has a large volume of branches that cover the country and may be able to provide the same assistance for them if RSPCA cannot.
Overall, there are options for this cat to receive care. Of course, we need to have it clearly established who is the owner at this point in time to determine the most appropriate option. So, if your son takes over ownership, the above charities can help him. If the people won't relinquish care or your son really cannot take the cat on, then the RSPCA would be the best option to ensure the cat got the help he needed and was rehomed to a family that could provide the care he will need over his lifetime.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
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