Blimey, been trying to post on this thread on and off for an hour ...
I was just adding that you should tell him now, or from the earliest. We made a book from laminated papers etc, explaining that mummy couldn't make babies with her eggs, so we mixed daddy's sperm with "A's" eggs and you grew.
Slightly different with us as dd is a straight surrogate baby, but in a way, the same.
We made it very clear that she wasn't given away by "A", but was an egg that she gave to us specifically to grow into a baby, that that egg would have just not been used of it weren't for us.
My dd was read this book and shown photos etc from about 1 yr old, we had photos of course of her being in pregnant "A's" tummy etc.
maybe, in all seriousness, you could do this, but find an Internet photo of an egg being fertilised in a Petri dish or similar?
Wish I was near you so I could show you DD's book. We leant it out to a local family a while bank and they made one for their dd.