Also, if he shaves that area, it might be a reaction to something he is using to shave, especially if he shaves on days he knows he is going to see you.
Aj
It happens every time after sex? If that's the case, I'd suspect an allergic reaction to something, not herpes.
Are you all using lube, or condoms with a lube? Perhaps its a reaction to that. It could also be something he is using to clean up with after sex, like soap, if he uses that, or detergent on a towel or washcloth, etc.
He might want to get this checked out.
Aj