Is there hope?

   so my boyfriend and I have been running into an issue recently. For this past month I haven't been able to cum during sex. When we first started having sex it was my first time and everything was so new and exciting to me. I used to cum just fine, but now I'm not. Any ideas?? He's getting really down about himself and I don't want my not cumming to be a problem. He's already stressed about a few thing. Btw I'm 19 and he's 25. He knows what he's doing, but no matter what I say he still thinks he's bad at sex now. Any help?????