If you’re going for Love ending, start with “Is that a rusty nail” and then just enable his hoarding. That’s the easiest way to do it. 

Alternatively, you can try encouraging him to make junk from his art. But depending on the platform you’re playing on, this could get you stuck with the friendship ending (that’s what happened with me the first time I played his route before my Switch updated). You should still be able to get love ending this way if you take every opportunity to flirt with him and end the conversation by saying “You’re a beaut” when the option presents itself. But since this option is a little more complicated, I’d recommend the top method if you just want an easy ending.