Test your water and post the results.
It looks like a bacterial infection. Have you ever noticed thin red threads protruding from the anus? It looks badly ulcerated.
do you have a quarantine tank?
separate him from your other fish. Treat him with a broad spectrum antibiotic that hits both gram negative and gram positive bacteria.
Look closely at his entire body. If he develops other red patches with a bright red line around it, he may have
bacterial hemorrhagic septicemia. A very nasty, fast moving disease often brought on by deteriorating water quality. It doesn't look like it to me from your pic, but better safe than sorry. Best treatment for that is oxytetracycline.
First thing I'd do is a decent sized water change in your tank.
Then move him to his own tank and treat him.
Hope hes' still with you, but he looks very thin...
