What is being told to you is: You don't have to be a biologically related to be a father to a child. But you don't want to hear that. The DNA test proves he was not biologically related to the boy, but it did not relieve him of being the father. Instead of spending x amount of years trying to prove he was not the father he should have spent those years trying to be a father.
We are not in your shoes, but trust me the ones we wear are the same size and shape.