Welp, so sorry about that. I haven't done that yet, and this is what I can only give you that might help you. 🥲
Okay, I think getting an "emerine crystal" will start the heart process. You're just gonna need at least one and talk to him about it. If you want to make it faster, just collect at least 5 to 10 emerine crystals instead of going back and forth to the cave.
This is from what I read about in the wiki. But now I have this problem whether I want to restart all over again or not because I didn't get a relationship with Gunnar. What's holding me back from doing so is the Bareshade expansion and the junkyard event... I just hate the rats.