I don’t know what to do

My boyfriend and I are both 19 and we lost virginity together... But we have this huge problem - I am not allowed to use birth control since I had hormone problems 2 years ago. Okay, we agreed to use condom and to have sex only when I have low chance of getting pregnant...but every time we use condom he can’t stay hard...only for a couple of minutes...he loses erection... I have no idea why is this happening?! Currently we do have sex, he puts condom but later we remove it and we make sure he never cums inside me. I am aware this is risky, I am so so stressed every time I expect my period, but I just don’t know what’s wrong...should we try other condoms?