Is he a pet or part of a meat rabbit breeding program?
That looks like an abscess to me, and I would suspect Pasteurella. If you have other rabbits, it would be best to cull (as in kill) him, since it can spread to your other rabbits.
If he is a pet and you have no other rabbits, you can attempt to treat it.
It will need to be lanced on the downward side and the pus expressed. The incision will need to be about 1/2" long to make it a little easier to squeeze the pus out. It will be very thick and pasty, not runny as in other animals. It is also very sticky, so once most of it is out, irrigate the wound with hydrogen peroxide to loosen the remainder and get it as clean as possible.<br /><br />__________ Tue Feb 05, 2013 10:59 am __________<br /><br />You may want to follow that up with injectable penicillin. I will look for the appropriate dosage if you decide on that route.