Thanks, Gina...sorry I couldn't get to the phone fast enough.
I wouldn't use tetracycline-and the abscess DOES need drained. It also most likely needs meds targeted for an aneroebic infection, which are most common in closed wounds-or abscesses. My choice would be either Baytril, Clavamox, or Clindamycin COMBINED WITH Flagyl (metronidazole), for at LEAST 3 weeks, twice daily-dosage must be based on the weight of the glider. Once a day will not keep it in the system with enough consistency to be successful. I'd highly recommend finding another vet-if we know what city you're in, we can help with that.
I'm going to post a pic of Lyah-my girl who had to have a tooth removed. If it looks at all like yours, let us know, or if different, try and describe how. 'Lumpy jaw', an infection in the gums/mouth, or an abscess, or an infected tooth, can ALL present looking this way. Hard food isn't required to cause it-all it does is increase the likelihood of such an infection through a scratch in the gums, but it's absence by no means means a glider can't get one.
I am also concerned as Dancing mentioned-if not drained, the infection can go systemic and kill, and in a shorter time than you think. An undrained abscess is next to impossible to kill (or get rid of), because there is no blood circulation inside and the antibiotics can't get in there (a decent vet would know that-and also know that a glider isn't like a cat, dog or more common exotic animal, and by the time symptoms show, it's late in the game).
The vet should also check for ANY loose or damaged teeth-if there is one, it will need removed in order for this to heal.
Here are 2 pics of Lyah-does it look anything close?