How much does he need to know?

Ok so I have been dating this guy for 3 months and it has gotten very serious very quickly. Is it normal for a guy to want to know details about my past and people the I have been with. It makes me uncomfortable that he is thinking about me with someone else. I dont ask him details about the girls he has been with because I just dont wanna know. I have been with more people then he has but I still dont understand why he would want to know every detail. I need advice!