Most Helpful Girl
I would maybe respond to the ex, but only after talking to the boyfriend about the whole situation first. As long as you explain how close you and your ex were (in a friendship-type way, of course), your boyfriend should be cool with it. Hear him out if he has any concerns, but also try to convince him that he seriously has nothing to worry about. Make it clear to your boyfriend (and your ex) that you only want to be your ex's friend again, and that you would stop talking to your ex if he ever tries to flirt with you. Your boyfriend shouldn't have a reason to feel like you're hiding anything, so be really open about the whole thing.
With all that being said, if you think that talking to your ex could possibly bring back old feelings, which honestly is bound to happen, then it's probably not the best idea to talk to him while you are in a relationship. If you were to catch feelings again, you would be creating a really messy situation that would only lead to someone getting hurt, which I'm sure you don't want.