Confused

Okay so I have a fwb (friend with benefit) and every now and then we'll have sex. And I'm really new at the whole sex thing because he was the first person I've ever had sex with. Recently when we tried to have sex it just wasn't working. I don't like the positions he likes, but we don't know any others that will feel comfortable for both of us. And I was just getting frustrated because I don't know what to do or how to do anything.