What Does A Rash On Penis Indicate?

my boyfriend was recently noticed a rash on his penis. He went to the urgent care and was treated for syphilis, 2 penicillin shots. We hadn t had sex in over a month and I have a clean bill of health. he claims that he was diagnosed and treated prior to us dating which has been over a year. He said that the doctor told him symptoms can reappear even after being infected. Is this true and should I be concerned? Thanks for any help