How do you lie about a father raping the mother? If he tries to find out, there is no record, just the moms (my daughter knows her name, just in case he wants to know that. They don't intend on keeping that from him if he asks.
I guess just leave well enough alone until faced with whatever he wants to know, deal with it then.
All he needs to know is that his mother was unable to keep him, but she still wanted him to have a good life, to be happy and loved. What would it hurt to just forget that you ever heard how he was conceived? And if down the road as an adult he discovers some ugly truths about his biological parents, he will have made the decision to go looking for them.
I do understand family secrets. My grandmother went to her grave never sharing hers. My mother accidentally discovered it years after her death, when we were getting the house ready for sale. It just made us love and respect Grandmother even more. As a very young woman, she protected her family with a beautiful lie that no one ever had a reason to question. It shows just how much she loved her family!
Bless your daughter for saving this child. I'm sure she won't do anything to hurt him unnecessarily.