Telling my 8 yr old ...

I'm 15 weeks pregnant and starting to really show. I haven't told my 8 yr old son yet that I'm pregnant. I'm having a hard time because I'm surrogating for a couple that are unable to have children. I'm afraid he will be upset because he will want a brother or sister and won't understand why I'm doing what I'm doing. Also if he asks me if it's me and daddies baby which it is it will break my heart and I'm afraid his. Anyone have any advise? I could sure use it..