Boyfriend does not want STD check.

First off, im 25 so adults only. My bf is all excited about my IUD and wants to cum inside me. Ive never let a man finish inside me before because I've never been on steady BC. I don't know what hes got. Is that horrible to say?? Is there a higher chance of infection if he finishes inside me? Dr said my partners better be clean because the strings can lead infection into the uterus, bad things, ect... He doesn't want to get tested. Any advice?