bf can't stay hard with a condom

The guy I have been dating cannot keep an erection if he uses a condom. Indeed we have only been able to have sex fully (meaning he finishes) one time with a condom. He has been tested for STDs and it came back negative, and he is going to be tested again, but i'm still nervous to have unprotected sex because I know that certain types of STDs are really difficult to detect, especially in males. What should I do?? I said that I would have sex without a condom if he got tested (I'm on the pill), but now I'm having second thoughts. Does anyone know anyways for an erection to be sustained with a condom?? Help please!