I doubt that this is an infection. More likely, it's just irritation from friction, etc. It's not that the antibiotic isn't working, just that there aren't bacteria for the antibiotic to kill. I suggest mild cleansing and moisturizing, and perhaps a visit with a dermatologist, who can diagnose and prescribe an appropriate anti-inflammatory cream. The redness itself means little--anytime there's a rash in a sensitive area like the groin, the skin can stay red for weeks, but then it goes away. I think he should see someone who knows what the rash is, which should set him straight and make him better. But I don't think he caught anything from you.
Take care.
Dr. Rockoff
Also, if it were a bacterial infection from me, would Flagyl be effective for a man, like it is for a woman's infection?