I wrote the above reply and forgot to register, this is my answer again.
He didn't kill Mary. By being "Not Right" means to me that he was different to the other kids and more than likely he was Autistic. Autistic people Never lie, so he was telling the truth. When he says "he left her by the river" he is not changing his story, he is saying in not as many words, that the last time he saw her, he left her by the river. That boy is innocent.