Frat/Sorority Party...?

There is this guy I like and we have been talking a lot. Well, he is part of a fraternity and he told me he was busy tonight because he had a party with a sorority. well, I know that in the past he's always invited me to parties that his friends throw and I know that his ex girlfriend is part of this sorority and I have a feeling that's why he didn't ask me to come or are they fraternity not allowed to invite non members to their group parties? I've had a feeling that he still likes his ex. He has told me that no he doesn't, he just wishes they could be closer because it's hard losing a good friend but I think they've been talking lately and Im starting to worry.Do you think he just didnt invite me because he plans on hanging out with her tonight?