There is nothing that you can do to make someone change their sexuality. If you were to use some sort of spell to keep women away from him, and it succeeded, you would be condemning him to loneliness, as he is not wont to suddenly have a desire in men for lack of women. He's a grown man capable of making his own decisions about the sort of women he wants to be around. So long as he isn't being abused, there's no reason to try and interfere with his lifestyle choices. Don't convince yourself that your desire to have him to yourself is for his sake, that's quite the toxic mindset to have.