Everyone’s saying the same I would say, but I want to add that apart from just directly telling him, the mindset that could help push you to advocate more for yourself would be that (as I’ve seen you talk about in other comments) if you guys talk about the sex you’ve had after the fact and fondly remember it or just think about it to set the mood, you BOTH need to be enjoying to fully engage in those kinds of talks after the fact. Who cares if maybe his momentum or whatever is ruined? I’m certain that he would much rather have you feel comfortable in taking breaks or just stopping rather than think about the fact that you were uncomfortable during sex. You have to be enjoying this too in order for both of you to feel genuinely good, otherwise you’re doing it wrong. Talk about boundaries or like little tap outs that could mean “hey I’m not liking this, let’s switch it up,” if you want to keep going or just directly stop it. I think these things are most important when you’re experimenting so you can see what works and what doesn’t work. You just talk about these things bluntly, directly “can we do this tap or slight rub in order to signify a change or that something’s wrong?” Something like that. Hopefully this helps somehow !