how can I teach him what I need

Hanna
Me and my boyfriend have been dating for 7 months and we have very passionate sex buti never reach the big O! And when I say never I mean NOT EVEN ONCE! Majority or the time I'm on top and still can't reach an orgasm.. Im starting to think he's just too big for me
 he has an extremely big and girthy penis so he has never really learned that it's more about the motion of the ocean than the ship! 
I'm not 100% sure what makes me cum in bed, but it's definitely not what we're doing! How can I teach him to explore new things or try new moves without offending him? I feel like this is a sensitive topic for him he's very reserved! 
Help help help!!!