WHY cant i summon up the courage to talk to him!?

there is a boy (ill call him Tom) who i knew quite well, cause he is my bothers best friends brother. i used to go to this sports thing that he went to and we got on really well. since i moved up to his school (hes in the year above me) i havent had the courage to really talk to him. even thought i dont know him that well, i cant stop thinking abut him. im not so bad anymore. he used to post on my wall on facebook all the time and i could talk to him on chat for an hour! he also used to say hey to me when we ped each other in the corridor. but face to face we havent even said anything ped "hey". he is really shy sometimes and hasnt got a mean bone in his body but i just cant get the courage to even say hey to him. i dont know why!! what should i do??