Typically, when you get an initial outbreak of herpes, it will appear at the site of infection, though that's not a hard rule.
Herpes infects nerve groups, and genital herpes infects the sacral ganglia. This means that you could get an outbreak anywhere in the boxer shorts area.
I think you should get this checked out. I'm not convinced it's herpes, and it's definitely not syphilis (at least not from this encounter - it takes at least 3 weeks for a syphilis sore to appear). It could be ringworm (not actually a worm, but the same fungus family that causes jock itch).
Since it's lasted this long without change, it's not likely to be herpes, but if the doctor thinks it is, ask for a type specific IgG herpes blood test. Don't let them do an IgM test - it's unreliable for adults and should only be done on newborns.
If it's positive now, it's a pre-existing infection. (If it says positive for type 1, that's a pre-existing oral infection most likely, as about half the adult population has it, and 90% will never get symptoms, like cold sores or fever blisters.)
If it's negative now, test again at 4 months. It can take that long to develop antibodies, though most will by 4 months.