I desperately need an AU where no one tells Damian he's Bruce's son.
They all assume that he already knows, because he always gets called The Son of The Bat, but Damian ends up thinking he's Jason's son. Why? Because Jason is the only other one around the league who has a bat print on their shirt.
Who always keeps wanting to have custody battles with Talia? Jason. Who guards him around? Jason. Who is literally always around, except for Talia? Jason.
When he does end up needing to go to Gotham, Damian probably just assumes he needs to go to his other, slightly better (Jason's words), grandfather for training just like with Ra's. Jason did, somehow, along the way feed into this, making jokes that he doesn't know Damian actually believed.
So obviously, when Jason leaves him on the porch Pre Red Hood identity reveal, the first thing that comes out of Damian's mouth when he first met Bruce was:
Damian: I am the Red Hood's son.
Dick: Okay, so I wasn't expecting that..
Damian: I was sent by my mother, Talia Al Ghul to train under you.
Dick: Huh. Okay, so I did expect that. Slightly.
They only test Damian for Talia's blood, and bada bing bada boom, he's hers. Bruce tries to call up Talia for this, and she only laughs on the other line. They do still keep Damian around, reluctantly training him for Robin. Dick was only glad Talia had moved on her "beloved".
Later, after Jason's identity reveal, Bruce revels in being a grandfather only for him to look confused and go:
Jason: What do you mean you're a grandfather? No offense, but Dickie doesn't seem like the type to have kids. Same with Tim. Doesn't seem like a Teen dad to me.
Bruce, slightly concerned: What about Damian?
Jason: ..No? He's Bruce's. Why would you think I sent him- Oh.
In short, Damian has the hardest revelation of his life later in the batcave as they try another DNA test. Jason is laughing hard in the corner, hardly breathing