Just use the unity console: https://www.nexusmods.com/pillarsofeternity2/mods/2
It will not break anything and his dialogues will immediately trigger at 50 (i think) and 150 points, the reverse if you want rivalmance.
Does rivalmance even go anywhere? For me it was just angry smooch followed by "No time for that, maybe later" -- "Okay then". Or you need to be extra boring to qualify?
Anyway, here's how to get Aloth to -2 without even trying:
Beeline to Neketaka from Port Maje, beeline to Tekehu, flattering him and encouraging irresponsibility whenever possible, that should net you a -1. Beeline to the bordello, talk to the bartender, support Tekehu all the time. Go upstairs, hire every employee (tip: the bad poetry lady has a conversation loop that gives you infinite diplomacy and positive points with Serafen and Xoti). Voila, -2.
Have to say, not a fan of this new Aloth and he's making me to retroactively dislike old Aloth, as I found out going back to PoE1. Now when I know he was probably eyerolling and sighing all the time in PoE1 as well but I didn't have the means to notice that. Kudos to artist for the portrait, though. Didn't like it at first, but it captures new Aloth's essence perfectly.