How do you feel about your boyfriend being friends with an ex?

I’ve been with my boyfriend for about 2 years now, but previously he had left me for one of his exes and they were together for awhile before we got back together. I just saw that he recently started following her again on social media and it doesn’t sit right with me. I confronted him about it and he said that he was just being nosy and he has no intention of getting back with her, but after everything him and I have been through (I’ve caught him talking to another girl in the past) I don’t feel comfortable whatsoever. I think it’s great if he wants to be friends with her, but I can no longer be with him if that’s the case because I do not feel like it is fair to me. What do y’all think? I feel stuck.