His mother admitted to him she kidnapped him and took him as her own son because she was sad and desperate at the fact that it was impossible for her to have her own baby.
I'm so sorry, baby! I know I did such a horrible thing! I was just so sad and desperate..
Is that why you wouldn't let me go outside and play? Because you just kidnapped me? You deprived me of my freedom and childhood!
After hearing the news, the child was extremely sad at first. Then after a while, he started to be angry and hateful. He realized he wasn't able to have a normal childhood and that he didn't even get to meet his real parents. He felt betrayed and violated.
Please forgive me! I love you so much and I did all that to protect you!
I'm not your son! You stole me from my real parents and made a sad kid out of me by denying me my right to play and be free!
He decided he could no longer stand being in the house. And so he left, leaving her mother devastated.
No! Please! Don't leave me! You're my only son! You're all I have!
He couldn't think of any other place to go but his one and only trusted friend/classmate. And so he went to this friend's house and explained why he ran away from home.
..... then I just ran away!
But isn't that still a good thing? I mean she still chose to tell you the truth no matter how hard it is for her.
Oh no! I must have made things worse..
He started to realize that his friend had a point. He regret leaving his home and mother just like that. She still raised him and cared for him just like he was her own.
It isn't too late though. I'm sure she's worried about you.