However, if you think that the frequency, degree, and intensity of his sexual exploration is a concern, then you might consider this behavior an unconscious cry for help. The occasional mooning of friends or show-and-tell with the boys on a sleepover is very different from his initiating sexual games with his male friends virtually every time they get together.
In general, I would not recommend that a seven-year-old be present for the birth of a sibling. Even if the birth was carried out with no complications (a sudden C-section, long and painful labor, etc.), I don't feel kids at this age are emotionally equipped to witness and process this very dramatic event. This is not a child watching little baby animals being born on the Discovery Channel as their parents sit with them in the living room explaining what's going on and assuaging their fears and anxieties. This is his mom in a hospital room (or at home) involved in the most emotionally laden experience possible. You know your son better than I could ever know him, but I would not favor his participation because of his sexual curiosity.
Book suggestions for him are:
Happy Birth Day, by Robie Harris
Flight of the Stork: What Children Think (and When) About Sex and Family Building, by Anne C. Bernstein
When Sex is the Subject: Attitudes and Answers for Young Children, by Pamela Wilson
Have a great birthing experience and congratulations in advance on your next child.