Why is he acting like this?

Unlike women, men tend to grow into a relationship and develop feelings toward a partner. By the sounds of it, he is developing feelings which are leading him to fall in love with you. Right now he sees you as his sexual partner not just a fwb. It is best you clarify with him that you intend to use him purely for personal sexual gratification and have no feelings toward having an actual relationship. The sooner you explain it to him and the more blunt you are when telling him, the better it will be for both him and you. He needs to understand he is a male prostitute giving it away for free and nothing more.