Lucifer tried this hallucination trick on Sam before but thinks it might work better on Jack. Kid's easier to manipulate since he's so young so this should be a piece of cake. And it helps that unlike Sam, he's not telling anybody about it. Works in his favor. Sam and Dean tricking Jack into the coffin was the best thing that could have happened for Lucifer. He'll get the kid on his side relatively soon. Just have to push him to the breaking point.
Lucifer can't wait to convince Jack to get him out of the Empty and resurrect him.