Had Unprotected Sex. Got Redness At Base Of Penis. Is It STD Or Allergic Reaction To Nuvaring?

I am in a monogomous relationship with my boyfriend. I use the nuvaring and he has never been with anyone who has used that before. We have been having sex for about a month now, and he recently (in the past week) has noticed redness at the base of his penis and minor inflammation. It is there one minute, then a few hours later it is gone, then eventually comes back. He is worried that he has an STD , but I have just been tested clean in November. There are no blisters, or peeling or scabs of any kind. I think he may be having an allergic reaction to my nuvaring. Is this possible?