There’s a lot of reasons why someone might ask you to stop, and the critical thing is that you do so immediately and check in.
It can start hurting - maybe you hit a wrong angle, maybe there isn’t enough lube. There can also be triggers associated with sex, psychological aspects where a stray thought, a brief smell or feeling, can make things not fun in the moment. Maybe you are pressing on your partner in a way that starts to feel uncomfortable or cramp. Being penetrated can feel intense and overwhelming, and sometimes it’s possible to slip from fun to not fun out of nowhere.
Making sure that your partner knows that you will stop if asked is critical to helping them feel safe.
It would be difficult to show off his boomstick that way though.