It seems to me, reading your sad, unsympathetic letter, that you took against this little lad and his mother from the start.
You've allowed Confirmation Bias to influence you ever since.
I'm not sure they ever stood a chance of getting into your heart and it seems nothing's changed in the intervening years because you were never prepared to let it. You never wanted him - and it shows.
Can you imagine what sort of hurt and sitress that will have caused him?
I have a step grandchild (I dislike that word 'step' even though some people think it is necessary) and he has a lot of problems because he has a narcissistic father who still negatively influences him.
My son has been the stabilising, positive influence in his life and it's made a difference, as has the support of his mother, my DIL.
My grandson also had problems at birth which have not made life easy for him.
But he's a loving, caring little boy and, like your grandson (or the child you choose not to call your grandson) he's hitting puberty and going through the usual difficult times all youngsters have with hormones, including bouts of disrespect and rudeness and more.
To you, this seem cause for resentment because you can't tell your 'grandson' off.
You are taking your resentments out on this child.
Is that fair or just?
Maybe your not wanting anything to do with this little boy is the best thing for him.
What he needs is an understanding, sympathetic grandparent who sees what he's had to go through in his short life and allow for the fact that he cannot always articulate his pain and hurt.
From what you've written above (and that's all I have to go on), I don't believe you are that person.