If they are just friends now then he should introduce you to her.
If he is not willing, then maybe you should question his honesty.

You can bring this up. You don’t have to come on to a forum and ask strangers what’s going on in his mind. We don’t know. It’s not necessarily weird for him to be in touch with someone he knows well and who was very important to him at one point.

You have some concerns about his relationship with this person, it’s clearly bothering you, so say so. Don’t accuse him of anything, don’t “question his honesty” or make a big scene, just say, “hey, I know we talked about this before, but I just have a funny feeling about this girl you used to be with. Can we talk some more about how you’re feeling about her and why you like talking to her so much? And can we talk about how you feel about me?”

If you just talk about yourself and your feelings, he’ll hopefully respond in a supportive and reassuring way. If he’s dismissive and defensive, then maybe that’s more reason to worry, and then you can tell him that’s made you worry more. If you can work through this together, your relationship will be the stronger for it.