How do I flirt with him and agree with his motives
Agreeing with Iliff's motives is during the walk through the city. This requires 2 devotion points to trigger. You need to choose the choice asking if hes jealous of Yukine.
Flirt with Iliff requires 3 devotion points and NSFW mode to be turned on. When you re-meet after he explores the basement, you tell him to get new clothes. A choice should pop up if you meet the requirements and you need to choose "It's cute, okay."