Bestfriend/exboyfriend is talking to another girl. what to do?

my bff was also my bf last sept we got close over the summer and stuff so we started liking each other. after a month of going out i broke it off cus he was too much for me (clingy, etc). but weve been like bff ever since. i think he liked me the whole time but im not sure. his ex asked him 2 prom abt a week b4 the date and he felt bad so he ended up going even though he told me that he rly didn't want to go. he was being kinda mean 2 her and stuff and i felt bad and told him he should. he ended up going. now i think he's starting 2 talk to another girl (who had a bf back then&now is abt 2go on a vaca) that he liked during the summer or his ex (that he went to her prom with) frm like over a yr ago. and i'd shudder when people would think that we go our or "aw" to a pic of us or something like that. but i dont want him talking to her but i cant do that. i wouldn't be right? b/c of her we hardly talk on the phone now. i think im jealous? do i like him? wats the rite thing to do? imisshim