With all honesty, the birds and the bees is the best way to go about talking about this.
 
Bring them for a walk outside where there are flowers, and show them the bees going about their pollinating. Make it interesting to them, ask them questions to see what they think about the bees and the flowers.
 
Try to make it a conversation rather then a dictatorial. Ultimately though, the idea that the bees collect seeds from girl flowers and then plant them in boy flowers creating baby flowers is what you are getting at.
 
This won't so much tell them "How he got into the womb", but it will give them food for thought, to help themselves.

Hi,
 
Here is a listing of books to help parents with this important discussion: http://www.education.com/reference/article/Ref_Resources_Families/
 
These articles also offer some tips you may find helpful...
 
When Kids Ask Tough Questions about Sex:
http://www.education.com/reference/article/Ref_When_Kids_Ask_Tough/
 
Talking About the Birds and the Bees
http://www.education.com/magazine/article/Talking_About_Birds_Bees/
 
I'm including some additional resources below to support you at this stage of your son's education and development. Good luck!