I know this is the part of being parents we never think about when we hold those precious newborns, is it? LOL
Anyway, I think it's way past time for your eleven year old to know about everything, and I mean everything. It really is time for dad to sit him down or go out for some guy time and let him explain the ins and outs of all things male and female. You should be there to offer any backup info when questions come up later - and believe me, they will. Your right to assume that other kids will tell him all kind of things and 90% of it will be wrong.
I had "the talk" with my daughter a few weeks ago because it was Sanctity of Life day at church and she didn't really understand the message from the pastor about abortion. Later that night she was asking me about it so as I explained what abortion was she brought up the fact that Jamie Lynn Spears and even her Nana couldn't help that they were pregnant as teenagers because God had decided "it was the right time" which is what I had always told her since she could understand about babies. Well, I decided right there in the car on the way to Panda Express for takeout that it was time she knew that they "could help it". It went very well. What was really neat was the next day she came back with a couple more questions after she had time to think about it. She asked how a friend of her's at school had been "made" since she had two mommies so we had a talk about invitro. Then she asked me since I had the surgery to not have more babies after her twin brothers were born, does that mean that me and daddy can't do "that" anymore. LOL I couldn't believe I was rationally discussing that with my nine year old daughter! But I do feel better that she heard it from me first. And I made her promise not to discuss it at school so that other moms had the chance to teach their children first.
so, get your game plan together with your husband, there are lots of books out there if you feel you'd have too hard a time explaining it yourself. Sit your young man down and let it all come out. It won't be as bad as you think. Believe me, you don't want him going into middle school next year without this information.