What Causes Difficulty In Maintaining Erection?

My boyfriend and I are trying to have sex,but he can't maintain an erection long enough to keep an erection or to enter me. He has no problem getting the erection though. He's only had sex twice and both times without a condom. My friend told me it's because he's not used to condoms, so we won't be able to. Is this normal and is there anything we can do so we can use condoms or do I have to go on birth control