My boyfriend can't stay hard enough to have sex?

So my boyfriend and I have tried to have sex but when we go to do it, he can't stay hard. I can get him hard when we're just making our or doing oral but he goes flaccid once a condom is on. He has a very thick penis and the condoms we had were kind of tight (I know we need bigger ones now) Could this be part of the problem? He's not a virgin and he said nothing like that has happened to him before but he hasn't been with a girl in over a year. I also wasn't able to make him come during oral but he said that he can when he masturbates. I assume he's just nervous and it's because he really likes me. What can we do to make this work? I know that acting like it's a big deal with him won't helpful so I'm hoping something more subtle will work. Thanks so much!