It's not just the mother, he has a right to know the other members of his family as well.  I found my real father and his parents and one of my sisters embraced me into the family even though my father was rather distant about having me in his life.  

It doesn't matter how the real mother feels when she robs him of his ability to know where he comes from.  You don't understand what it feels like to not know where you come from, and the relief when you finally figure it out.