I've been trying to build up a friendship with him with multiple saves, trying different sims. My were was doing the Lone Wolf aspiration so she already had to seek him out and defeat him and they had a negative relationship. Even after taking a ton of photos to get their friendship up, there only seems to be a few hours to talk to him or attempt to get his mood up before the switch flips. He attacked her and of course the friendship she built up tanked again into the red.
Then I tried as a vampire which I knew would be even harder. But this vampire is very nice and has positive character values from childhood. She isn't mean to anybody. Greg attacked her on sight.
Then I tried with my mermaid/spellcaster family. I sent my spellcaster over to make contact. I took a bunch of pictures, got his friendship up, but he was still very angry and it was difficult to have a conversation with him. But my spellcaster has maxed out everything and I had him use "Beguile" on him to get Greg flirty and eventually got him to tell part of his story. I had to return a few times to get more of the story. I haven't gotten it all yet because he gets ragey after a few hours so I know just to leave once you can't communicate normally with him. So I think my best bet is with the spellcaster. I might try with a non-occult too but spellcasters and mermaids have given me the most success with building up a friendship. I'll keep at it.