What Causes A Red Ring Around My Penis?

Hello! My boyfriend has been complaining of a red ring around his penis. At first it was swollen but it went down. Now he has the ring w no swelling or rash. He says he has no discharge or itching. I get UTI a lot but he has never been diagnosed w one. I got checked for stds and I came out negative for everything from chlamidia to Siphilis. He has been tested for stds, as well. that came back negative. Can you help?