How to get ex back from rebound relationship

my ex is seeing another girl as a rebound ( a few weeks after we broke up )and we started talking again a week ago and I want to try and work it out again since I think our breakup was a mistake and it was over something stupid. I doubt that he would go long term with this girl and I miss him and I’m going to talk with him tomorrow about everything but yeah It’s annoying me that he’s trying to seem like they’re close when we all know she’s temporarily ( no offence to her )