I don't think pus even needs to be inside it D: If there was once an abcess in a place, you're likely to keep getting one there until you get some really exact antibiotics and maybe surgery to remove all the flesh that's been tunneled through by pus - after a while, the insides of the tunnels heal, and you've just got warm, damp pockets waiting for bacteria to get in. (my mum is diabetic and used to have abcesses in her gums and on her leg all the time - they virtually vanished after she gave up smoking, so try hiding Twiggy's lighter.)
I'm sure if you can keep draining the yuck out so that it doesn't build up and cause pain, he'll be fine. Rats aren't quite as bothered by having scabby, stinky bits on them as people are...