Poor bunny!
When I have dealt with abscesses in my rabbits, cleaning out the wound and treating it with raw honey and/or Neosporin (not Neosporin Plus, just plain Neosporin) has always worked. Be sure to clean it very thoroughly - be pretty aggressive about scooping out every bit of stuff in there, even if you have to open up the wound a bit more, or the infection will be likely to come back. Neither raw honey nor neosporin will hurt the rabbit when he ingests it, and he will, as he cleans the wound. You'll need to reapply whatever oitment you use 1-2 times a day because of this. I don't usually bother with bandaging/covering the wound, because I've never had a rabbit that was unable to get the bandage off within minutes.
I'd also keep the rabbit in a scrupulously clean environment because he will be using his feet and nails to clean/groom the wound area.