In my personal experience (which I have far too much of), abscesses only cause pain when they are pushing on something and/or swelling. If the pressure is relieved, so is the pain. I don't know if this is the same or not with guinea pigs. Relieving the pressure involves lancing/draining the abscess. Antibiotics are a definite must in treating an abscess. I wouldn't say that diagnosing "infection" is wrong for an abscess; I would think of an abscess as a type of infection. It's a collection of pus and gunk, so basically, it's a walled off pocket of infection. I don't understand the choice to remove antibiotics. Even if they are running an analysis to see what the microbes would be sensitive to, I would have thought they'd leave the piggy on something in hopes that it might be at least somewhat effective, unless it was causing some sort of reaction or side effects.
Have they gotten the microbiology culture report back, yet? I know it can take a few days to a week to find out what grows ... Here's to hoping that they'll be able to get him back on antibiotics and healing soon!