I think you need to go to a RABBIT specislist vet immediately and see what the prognosis is. The way you describe his abcess I think you may be doing him a disservice by not getting him treated or medicated. And just because you don't think he isn't in pain, does mean he isn't......that's why the jerks doing medical research use rabbits.....unless absolutely terrified for their life they have no voice.

Not all vets take the medical training if rabbit care and what meds you might give a cat or dog can kill a rabbit. It is possible for abcesses to be drained and you may have to keep draining the abcess every x number of days for the rest of his life. Or it may be time to say goodbye. Either way you need to see a vet ASAP. See if your state has a local chapter of the House Rabbit Society. Or go to the HRS web page and check the Vet link www.rabbit.org