Worried Sex

Hey guys so I’m not really sure how to even say this but I’ had sex with my boyfriend and it was his first time and all was good. We’ve been having sex for a few weeks now but every time we start hes hard and at some point in the middle out of nowhere he becomes soft. Is there something I should be worried about? Is it performance anxiety? Is it because he doesn’t find me attractive? This keeps happening every time. Were 20.