He seems to be having an affair with many girls, including me. What should I do?

Try to make him feel only for you. After all, you must not miss meeting someone you like. Tell the truth about yourself. Maybe he likes you and has an affair with other girls. Maybe he likes you, too, and wants you to be jealous. Anyway, all you have to do is tell him your truth, and the ending doesn't matter. As long as you fight for it, there will be no regrets in the future.