Why does my boyfriend always go limp when i go on top during sex?

i am 20 and me and my boyfriend have been going out for a while now and have only just started having sex, i dont really understand having sex can you tell me some reasons of how i could makee it better for me and my boyfriend? and everytime i go on top he goes limp but when he is on top of me he doesnt go limp, am i doing it wrong? Pleasee answer!