I think the reason he broke up with her is because he can feel that he started to love her and it will be hard for him to leave her if he continued their relationship. In the recent chapter he already told her that he also don't want to get married that's why he said that Na-whi is her ideal girl because clearly Na-whi don't want to settle too. And i think his a playboy type of guy..and maybe it is his first relationship that he fall in love with the girl that's why his afraid. And clearly Na-whi started to fall in love with him too that's why she's also hesitant to let go..but still i don't like his way of breaking up..
The update is really early today..thank you..