Being a guys leftovers?

i am seriously in love with this one boy at my school. i think he likes me to cuz he always IMs me and calls me everyday, and at school he always makes me laugh and sits next to me at lunch. well we are both in 7th grade. he is supposed to be in 8th grade thoguh. the thing is, he like this one 8th grade girl who he thinks likes him back. he always talks to me about her and says he gonna ask her out. well im pretty jealous right now cuz after becoming so much closer over the past week, he just wants to leave me for another girl (we were never going out, it just seemed that way to me) so i dont want to end up as his leftovers, (if she rejects him, he asks me out) or do i? is it ok to be the leftovers, i mean i know it sounds bad, but i really like him and i know he likes another girl more than me, but if something makes him not love her as much, should i go out with him or not? plz help, girls AND guys