I had a fistula ten years ago in the midst of (or caused by) the worst flare I ever had. He should see his GI again. It sounds as if he might have an abscess if there was pus. If so, he needs to have it drained as well as antibiotics. A fistula isn't painful (at least mine wasn't). I also had an abscess about
4 years before that. It was very painful.
If it is a fistula, there are also medications that help close it up. I was on cyclosporine which made a difference. Since then (1997), infliximab has been used with success. In any event, he needs to see the doctor again.
By the way, I had been told that if you have a fistula and/or an abscess, it is a sign of Crohn's rather that UC. That is not necessarily so. I have UC.
Diagnosed UC 1975.
Azulfidine Entabs 500 mg. 4 twice a day
Azathioprine 100 mg per day
Colocort - as needed
Folic acid, vitamins C and D, multivitamin, metamucil, fish oil, turmeric
Hospitalized 1997 - intravenous cyclosporine.