re: Oher blindsided, gaslit by Tuohy family; wishes he had gone to Tennessee

I'll have to go pull the book off my bookshelf and see what it says (I trust it more than the movie), but in the movie when Leigh Anne and her friends are having lunch at the country club, one of them says to her
"Well, I say you make it official and just adopt him."

Her response is "He's going to be eighteen in a few months it doesn't make much sense to legally adopt. "

So taken from that, I was never under the assumption that he was adopted. They were just named his legal guardians/caretakers.