Another way to think of it -- you let him know how you're doing, he decides what to do with that information. That's true whether you choose to phrase things in terms of rules, boundaries, guidelines, agreements, needs, or none of the above, because in the end your partner always retains free agency. So, you can say to him -- "I wanted to let you know that I think I am in a place emotionally where I'd be ok with you spending weekend time with Shasti. So, if you want that, know that you can go for it without worrying about me. If it turns out that I was wrong, and it affects me more than I thought, I'll let you know."
Nothing in there about what he should or shouldn't do, simply info about you that will (hopefully) be relevant to his decision-making process. Do you think that might feel better?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.