For the "Spicy Ending" you have to invite him over to your apartment on the first day and then invite him to sleep in your bed with you. Then I called out to him and kissed him. From there, if you allow the NSFW choices to happen, you get the option every day on. It seems that if you miss that one, it might not open for the days after.
I'm also pretty sure your choice of enabling/disabling NSFW scenes and your choice of being 18+ or under 18 in the very beginning will change whether it allows you to access those scenes or if it goes with a more platonic route based on that choice (if that makes sense)
Let me know if you want a more detailed guide and I can do that for you :)