I want to start the dark brotherhood questline. In keepers say nothing about the boy so I need to speak to Maul.
Maul is not to be found at the Riften gate and also not in the black-briar manor.
Can anyone tell me where I can find him?

If he's not at the gate, check the docks by the warehouse.
Alternatively, just go to Windhelm and walk near a guard; they'll mention it and your log will update.