What Causes Penis Burning Sensations After Urination?

My longtime boyfriend, he's 53, who happens to be a doctor, got a burning sensation in his penis after unrinating. this was about six hours after sex. He says his symptoms is "almost exclusively STD related," and more specifically, from a sexually tramitted disease. He asked me if I am having sex with someone else. In three years I have not had sex with anyone but him. I asked him what else could it be because I have not been with anyone else. He's a little angry and a lot suspiciuos and I can tell he doesn't believe me. Could his burning senation come from my having a yeast infection? or some other infection? I don't have and have never had gonnorhea, chlamydia, trich... I used a new soap, could it have come from a new soap that I showered with before sex? It's not prostatitis because he had prostate surgery. could he have some kind of urinary tract infection? I will get tested to prove to him that i don;t have an STD. But what could this be? He's not talkingn to me much right now. Thank you.