Some parents keep it a secret that a child is adopted, but they find out on their own. It never leads to good outcomes.
It can lead to feelings of betrayal and shock, identity issues, trust issues, anger and resentment, feelings of rejection and abandonment, and impact their self esteem, according to ChatGPT.
Given the current developments, it wasn't a good decision on the part of the Touhys to have kept it a secret. They should have let Michael Oher know early on that he was not their biological son. Now that he's found out on his own, he's predictably acting out.
Child psychologists see this all the time.