Relationship and Exes

Okay so me and my boyfriend have been dating for 7 months and he’s constantly texting this one ex. and we have had disagreements about this and we got into a argument and he told me that if I wanted him to stop texting her completely I would have to leave, and today he calls his sister because her 21st birthday is this weekend and she asks him if he’s gonna bring his ex instead of me. I don’t know what I should do. I wanna talk to him about it but I don’t want it to be an argument. I’m tired of him texting his ex and etc. It gets under my skin, he also claims that she respected the relationship but at the beginning of our relationship she also said that she wanted to get back with him. I need advice and opinions on what I should do and how I should handle this situation