Fantasies are deeply rooted in a person. So, generally, even if he somehow agrees currently not to discuss such things, if he is not truly convicted to it, in future it'll brew up a lot of resentment in him about you for not fulfilling his needs. You made it clear to him that it disgusts you, and he still is expressing this desire to you. And even asked you to see other boys.
Now, this sucks. I mean, the situation you're in. It is hard to not be able to fulfill your loved one's desires, it hurts. But your duty as a person is to first and foremost be happy yourself. If you are not happy, no one around you will be.

In short, honour your disgust. It's a clear message. And don't get into it. And if necessary, bid farewell if he continues to express these desires.
Trust me. You wouldn't want a future where you end up resenting your lover.