okay, i am going to be honest here.
guys are simple people. and you told him that he could dance with any girl you wanted. being a simple person, he probably assumed that you wouldnt mind. and honestly, you wouldve been bothered by any girl he danced with.
im not saying you're to blame, but you have to be straight forward with guys, especially if you feel insecure in your relationship, because each little thing you feel insecure about will eat away at you.

or, im completely mistaken, and your boyfriend is a jerk, and thats something that i couldnt tell you without meeting the guy.
i hope ive helped in some way.

Thank you(: It just makes me up that he even asked his ex to dance with him. And we haven't been talking since then. :/ But thank you for all your help!