Cumming inside me

This is probably a weird question but I wanna know how to get my SO to cum in me without directly saying those words. I would like for him to just do it on his own. He’s done it before by accident, neither of us had a problem with it, but I became addicted to the feeling. I’ve been trying to figure out what I did the few times he’s came in me that made him not be able to hold it in anymore. I don’t want to lock my legs or anything because i don’t want him to feel like I’m trapping him or something lol. Any tips on making it extremely hard for him to pull out?

(We’re not trying to get pregnant but we have discussed that we wouldn’t mind having a baby. We also have agreed on consensual non-consent. Just clarifying before any of *those* type of comments come in.)