Since he's your neighbor, make him notice you. :P Wear showy clothes when you go to take out the trash or go to the corner store or something .. like that he's eyes will be on you.
Make all his attention go to you when ever you're around him ... make him think over why he's with his girl, since you say she's not right for him, he'll prob compare you to her in his head at some point.
how do you know she isn't right for him? who are you to make that judgement! Has he said that he is interested in you? If he hasn't even noticed you move on! why are girls so hung up on trying to steal other girls boyfriends? How would you like it of some girl tried to do that to your relationship? your under 18 and still young go find your own man and leave this one alone...plenty more fish in the see!
Harsh I know but if he is flakey and leaves her for you wots to say he won't do the same to you?