I gotta be honest OP, I wouldn't date someone who had a history of hiring sex workers, nor would I date someone who was comfortable pushing you levels of comfort the way you're describing. I don't know that I have advice, but I definitely don't see you being unreasonable here. He knows it's a touchy subject, you've raised concerns and he isn't taking your feelings into consideration.

That said, if he's gonna sleep with prostitutes on a vacation to Thailand while he's dating you, then he's going to sleep with prostitutes on a vacation to any other location, while he's dating you.

The location isn't the issue. You don't trust him not to cheat on you. And, unfounded or not, that's the situation you need to work on resolving with him, or you need to walk away.