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
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