Damage control from jealousy

A bit of background: my boyfriend and I have been together for five years. When we first started talking, he was also talking to one of his coworkers. This became a constant issue with me because whenever he mentioned girl coworkers, I worried he he would be interested in them or have a coworker as a back up if we broke up. Needless to say, we had a lot of arguments about the coworkers he would casually mention in stories to the point where he avoids mentioning girls all together. He would say that “he didn’t want me to get mad” so he just avoided it completely.

Fast forward to now, I’m in a much better place than I was before. I’ve worked on myself and feel a lot more confident in our relationship. Now, my question is, how do I get him to be comfortable telling me about the women he works with? I don’t want him to feel like he needs to hide things from me. It just bothers me that he doesn’t feel comfortable. I know it’s my own fault for the way I would behave previously. So how can I approach the topic moving forward?