I'll address the worries in your question in a second, but first I want to check on some things about this relationship in general. I'm assuming this is the same boyfriend you've had for awhile now. If so, are those things you mentioned in previous posts--him yelling and scaring you, saying hurtful things to you--still happening?
With these worries about his ex, there are a few things you can do. The first is to ask yourself why you're finding it difficult to trust him about this. Is that fear coming from feelings about how you measure up to his ex? From ways he's acted in the past? Something else?
The next thing you can do is put blocks in place on social media to stop you from checking on her. That checking is only serving to make your anxiety worse, without giving you anything productive to do to address the underlying problems. Likewise, if you're still checking his phone, that needs to stop too. Both because it's not respectful of his privacy, but also because if you feel you truly can't trust him that much, that's a sign that there are things in this relationship that need addressing, and putting your energy towards those will be more helpful to you in the long run.