The first thing to check is: when did you have your last “boundaries” discussion with him?
If it’s more than a month, it is possible to sit down again and have an open heart discussion about this.
If you want a chat, instead of saying: “When you interact with her, I don’t like it and I think you should stop…”
“Let’s chat about exes today… How close do you feel it is appropriate for them to be? For instance would you be ok with me exchanging text messages with my ex in the evening while you and I are together?”
“What do you feel is ok or not – What’s your opinion? I am really interested in knowing what you think…”
Remember that you don’t tell him what to do.
When you have this chat about this, keep it short and targeted.
Another opportunity to practice your favorite mirror tactics 😉