The Dan CG is amusingly one of the easiest to unlock in the full game, but it's hard in the demo because it's hard to get close enough to Dan in the limited timeframe.
Visit Q&A as soon as possible, take opportunities to raise your confidence (make choices that let you come out, visit Q&A, or have conversations about your sexuality), and IM Dan once you have the chance, which will unlock a planner option to hook up. Hooking up with him unlocks the CG.