Curate, connect, and discover
Martha and Thomas Wayne? He's met them, personally. Shook their hands.
Duke has known since his powers manifested that he can see ghosts and interact with them. The ones on both the visible spectrum for the living and those that are not, like Martha and Thomas.
He hasn't told Bruce, though.
Martha and Thomas don't want him to feel like he's being watched all the time in his own house, they know how their son gets.
Point is, Duke knows a ghost when he sees one.
So when he goes to school and sees a new kid on the campus, he knows this kid is both on the visible spectrum for the living, and also is very, very dead.
Poor kid just doesn't realize it yet.
Duke sets out to make the dead kid feel at peace so he can move on or at least accept what happened to him.
Danny's just startled that this random guy decided to not only befriend him on his first day of school, but in ghost terms, is declaring his undying love for him.