Suggest Treatment For Rashes On Head Of Penis

JUST RECENTLY WHEN MY BOYFRIEND AND I HAVE ANAL SEX HE GETS A RASH ON THE HEAD OF HIS PENIS IM RECOVERING FROM SECONDARY SYPHILIS HE WAS TESTED AND WAS NOT DIAGNOSED WITH THE STD. IVE HAD IT FOR THE DURATION OF OUR RELATIONSHIP AND WEVE HAD SEX MANY TIMES, BUT HE JUST RECENTLY STARTED BREAKING OUT IN A RASH THAT BURNS HIM DURING INTERCOURSE WHAT COULD BE CAUSING THIS?