What Is The Cause For The Burning Of The Penis After Unprotected Sex With Negative UTI's And STD's?

My Boyfriend and I are having unprotected sex. We were both tested for STD before having sex with each other. what it seems like is that every time we have sex his penis burns ( nothing happens to me ) and he is always asking me if i have a uti or any type of infection ( due to some women do carry ) I made an appointment with my Dr. advised of the symptoms he was having. He retested for STDs and that came back clear. Then He tested for Uti and infections and that came back clear. So now I am dumbfounded cuz i do not have a uti or any infections in my vaginal areas been tested for stds twice and everything came back negetive. I have no stds and no infections of any kind. But i do not understand why this is happening. How many times do i have to go to the dr for them to test me and tell me i am clear. I do that to give me peace of mind that i didnt give him any infections during sex. Is there any other possibilities that there could be since i know its not me.. Is there a possibility that he is allergic to me? I just want to understand why this is happening. I never had this problem with past relationships.. please help me?