Don't be surprised if the abscess bursts before you get him to the vet. I had that happen to one of my cats - Bozo had an abscess on his back (he actually looked like a dromadery camel it was so large), and while we were waiting to take him to the vet, it burst. It looked gross, but I think he was happier with it gone. When we got to the vet, they just cleaned it up and gave him some antibiotics.

Kiki has had a couple, too. The last one, I just squeezed the pus out until I got blook, cleaned up the wound with hydrogen peroxide and gave him some Keflex. It cleared up in no time.