Hello, I'm Dr. Chris Smith and I'd like to help!
I just need to ask you a few questions to be well informed and give you the best advice.
Was there an injury that you know of?
Does it get better then recur?
Does he go outside?
Cats don't generally create self inflicted wounds like dogs do unless there is a reason.
I am worried there is an underlying reason for this behavior such as a cancerous growth, even though it looks like a sore or a foreign body/inflammatory reaction in the area.
I would recommend surgically removing the lesion and submitting it for biopsy.
Has this lesion gotten better with cortisone and antibiotics?
Typically allergic type of lesions respond nicely to those treatments.
If those treatments haven't worked a biopsy is important, especially in a 13 year old kitty.