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.
I don't think they will tell him about the rape, as you say it is not necessary.
He will know his mother did love him enough to stay off her drugs during her pregnacy, and give him to a loving home. He knows that, and seems to be very stable and very well liked in school. Time will tell and pray for the best. (he won't know about the drugs)
I will tell you another beautiful thing about that family. It did not look like they were able to have children of their own, so I suggested adoption. They did. Then one year later she conceived and had my only granddaughter. wow
How the Lord does work. It's like HE said: I will give you your daughter, first you must take this boy child. Awesome present & she was born on Dec.31st, 14 yrs ago. Merry Christmas to you.