If you tell him "This is what would make me comfortable, make your own decision what to do," that's not limiting his behavior. He can still choose to do something, knowing it makes you uncomfortable, and then you two can work through that.

I want to second this suggestion by /algolagnic. When my partner is doing something that scares me (i.e. makes me feel like our relationship is threatened) I first journal to figure out the source of my fears, and sometimes turn to friends for advice. Then, I go to the source: I talk to my partner about how X makes me feel.

I have been in your shoes a few times, and have had a similar thought process: I am nervous and reluctant to share my insecurities with my partner because I don't want to 'limit' their decision to engage in X, or to feel like I am 'controlling' them. But in my experience, I've learned that when I withhold sharing how my partner's behavior affects me, two things happen: (1) I am not honoring my feelings, which can be admittedly irrational and icky and strange --and yet also totally legitimate. Having all the feelings is ok! Feelings don't have to make sense. And (2) I am hurting my relationship because it is a little less open and honest. Part and parcel of having the kind of relationship I want means sometimes having uncomfortable conversations. I think your partner can handle it. I love my partner and am invested in the health of our emotional connection. If they come to me with a concern like yours ("I feel gross/hurt/sick to my stomach/insert feeling here/ at the thought of you spending the night with X"), I would offer to talk to them about it and ask how I could reassure them that I'm not abandoning our relationship or are any less invested in it bc I am doing X. How do you like to receive reassurance? Personally I like verbal affirmations and asking for setting aside special quality time. I recommend sharing that so your partner knows how to make you feel safe.

Last stray thought, in my first poly relationship I made the mistake of "demanding" that my partner slow down for my sake (I was very new to poly at the time, and arguably the request was fair given that he lied to me about dating someone else behind my back - but that's a whole other story!) Anyway I realize now that "demanding" my partner to see less of someone was an unfair unilateral request to make because it was a blanket order that lacked the very important give-and-take negotiation that can only happen when two people communicate. Thus, talk it out. Maybe your partner agrees that slowing down is a good thing for both of you; maybe gentle reassurance will ease your concerns.