There used to be a book called "Where Did I Come From" which did a good job of explaining the sex act for kids - My son is now 19 so I hope the book is still in print. If not, check out the library or your bookstore.
I don't think it is helpful to say that a baby is born only of a mommy and daddy's love - it doesn't explain the people who don't have kids (especially those who want them but can't have them) - you don't want your son to say "Oh, they must not love each other." It also doesn't cover the same sex couples. Some of those couples adopted, but some of the lesbian couples may have chosen to have one of the partners become pregnant and give birth. I think it is perfectly acceptable to say that there are a number of ways for this to happen and that, for those who want to but can't, there are things that the doctors can do to help it happen. No matter how it happens, it is important that every child have someone to love him or her. You are already exposing your child to the glorious fact that there are all kinds of families - 2 parents, 1 parent, same sex, opposite sex, and there are also foster families and grandparents raising kids. It's great that you are celebrating diversity and trying to find the right words now to allow for all possibilities!
Enjoy your little scientist!