It's good that you are going to the vet, and hopefully a vet that is knowledgeable about treating guinea pigs. I've had a couple of pigs with jaw abscesses. How well they did really was down to the knowledge of the vet (and the knowledge of me as an owner understanding what was the standard of care and what aftercare was required.) The pig who got good vet care did very well and made a full recovery, the pig that got shoddy vet care unfortunately developed osteomyelitis and passed away. Luckily, with good treatment, these kinds of abscesses are treatable, so be assertive in making sure your pig gets the right treatment from the vet!
Guinea pigs form very thick pus and are prone to abscesses. Simply prescribing an antibiotic is not enough to clear an abscess in a guinea pig. The abscess will have to be opened, lanced, and drained. It should be left open rather than stitched, and it's very important to flush it out with sterile saline (or a cleaning solution from the vet, mine was kind enough to give me a huge bottle when Sundae had a major abscess opened!) This will help to clear out any pus that is accumulating again, will keep the wound from healing, trapping pus inside and starting the abscess all over again, and will allow it to heal from the inside out. This can take some time, so keep on flushing! It's also important to get a good antibiotic that is effective for abscesses. Baytril is often given to guinea pigs, but really is not much good against abscesses. Some vets will tell you that this is all that is available, but that's not actually the case. Depending on where you live, different meds may be available. Zithromax is supposed to be the best for abscesses, but it's not commonly given in all areas. I hear good things about Marbocyl, and Sundae did very well on chloramphenicol following her abscess op.
Lots of luck and definitely ask questions if needed... I found this forum extremely useful when Sundae went through an abscess operation and credit her recovery in part to the great advice I got here about aftercare!