Which antibiotics is he on?
Levaquin? More likely nowadays.
If he's on Keflex thats a problem

Surgeons put everyone on Keflex for the past 30 years, which is why most bacteria are resistant to it.
Antibiotic therapy needs to be directed by cultures, not ust guessing.

If the source is the mouth they may not be optimally effective.
Theres too many resitatant bugs to guess and being wrong can kill you.