Pretty sure emotion is not involved in this scenario.

Whatever extra it is he is getting from the porn that he paid for, it's not about emotions.

But you are right with the concern that it may evolve into paying for actual prostitutes in the end.

My suggestion: Find out what he would need in real life that would satisfy his needs for sex enough, without having to go to prostitutes.

Like, what kind of sex he'd like with you, and if you can find a way to give that to him, if he can also adjust to you to make you comfortable with it and so on.

Unless he's the kind of man that just has to cheat because they like variety that much. Those exist too.