The only scene that's as close to me petting him is a scene where the MC caressed his cheek while Asterion layed his Head on the MC's lap. Would that be it? 'Cause If It is, I've replayed the game a shit ton of times for nothing 🥲
When he is halfway healed, he says you can choose his fur color. Ask him what color he wants. There will be an "I won't hurt you" choice and petting his face.
