T.
As the mom to four boys, I certainly feel your "pain". We have an extremely open base of conversation in our home. There are no subjects off limits... Regardless of what it may be if the boys want to discuss something, we do. Even if I develope a "twitch" over it :) . Our boys range from 13 to 19 months and trust me, with the exception of the baby, all of the older boys(13,11,9) have really given me a run for my money sometimes. But we always answer their questions as honestly as we can. If they know they can talk to you without being judged, lectured, criticized, they will...
For us, I have found that being totally honest with them is the best approach. They get only slightly censored versions of the truth. This way, they feel like they are not being given the "run around" in my or my husband's answers and they are not embarrassed about having questions. My older two boys already have a full understanding of the "birds and the bees". And they feel very comfortable about vocalizing any questions they may have. They know that we will give them honest answers.
This is not a "one parent" thing. I think that I am actually the one that the boys talk to the most when they have questions; now when they are talking about girlfriends (yeah I know, girlfriends already!)and doing the "bragging" thing my husband gets most of that.
So, my suggestion would be to be open to anything and not dismiss his questions. Let him know that his curiosities are valid and give him an honest if censored answer. I would highly advise that you not tell him "when your older" or make him feel ashamed or embarrassed about his feelings. Just approach it like any other conversation, even if this one is a bit more uncomfortable for you.
Good Luck!
Y.