*If* it’s Eikenella (is that an actual diagnosis you’ve received or speculation?), vancomycin won’t really do anything for it despite being a big-gun antibiotic. Mechanism of action works for gram positive bacteria, but it doesn’t work on gram negatives (that’s what eikenella is... has to do with the cell wall buildup of the bacteria). A really dumb analogy is throwing napalm on a wall made of stone... it’d work great if that wall was made of wood, but it isn’t.
That’s a bacteria that resides in everyone’s mouth... it’s totally harmless unless it’s in the wrong spot. It’s the equivalent of what e. Coli is to poop. Dunno if he developed some kind of cut and sucked on it or was chewing on his finger but those are possible ways you can get infected by it. I doubt he was shooting up with dirty needles or having someone else bite him but hey those are other ways to get an infection from it. Diabetes makes everything worse. Simple explanation is that diabetes = higher sugar content in the blood, and bacteria love sugar for energy so they thrive more easily. Infections are more likely to occur and tend to be worse in diabetics.
Bottom line... at this point, you need to know for sure what bacteria the infection is being caused by and use an effective antibiotic. If there’s a pocket of pus, it likely needs opened up and drained. Keep his blood sugar under good control throughout—it’ll be even harder to control than normal if he has an infection, and it’s more important than normal to keep it controlled. Good luck