Sex & relationship advice!!!

I’ve been with ol dude for a month now, & when we first had sex we both noticed this red rash type thing on his shaft near where his balls are. We just brushed it off as possible friction irritation. But today, we had sex and I was looking at his penis and it seemed to have gotten a little worse.. he asked me (jokingly) if I had given him some thing and I said I don’t think so, I’ve been tested twice this yr and both panels were negative. I want to go to the doctor and he told me he was just kidding & that it probably has to do with a friction burn or whatever. I’m still going to get tested but my no no square feels fine, and seems to be healthy, I haven’t noticed anything on myself.. question is.. has anyone seen or know what I’m talking ab? If so what do you think it could be. Maybe he’s still messing around with other girls?