Ex boyfriend's behavior?

my boyfriend and i had been dating for 7 months when he suddenly called it off a week ago. before we met he was a partier and tried getting with lots of girls. then he changed to be with me.. the day after our break up he asked to see me and told me he didnt wanna be without me and wanted to "unofficially" be together and wouldnt hook up with other people.. the next day i find out hes texting sl*ts asking them to sleep over and hasn't been sober for days. i was with him last night and saw him texting literally the biggest wh*re in my town (someone he talked to before we dated but found out was crazy). why did he just go crawling back to all these girls trying to get a hook up when he wants to be with me? and he's going to college in the fall which is why he broke up with me in the first place even though we talked about staying together for atleast the whole summer. any thoughts about why his personality and behavior have changed so quickly?