I have a problem, my boyfriend is so shy around me, but he will call me or talk to me alone..y? guys plz help!

ok, my boyfriend is THE nicest guy on earth, but at skool he won't talk to me, he won't talk to me if he is with his friends, i try to tak to him, nothing, a short "ok" or like a 3 word response. when i talk to him on the phone, he still seems shy, we have nothing to talk about, y?? and what can i do to change it? we have our share of inside jokes, but he will prentend like he doesn't get it when he is with his friends. HELP!