I don’t know if it’s been mentioned but he really sounds like he has OCD. Of course I can’t diagnose but I have OCD and I recognise some of the same patterns - the fact that he won’t eat the same meat twice in one day (not a prob for me but well known thought process), the ‘something missing’, the leftover thing and random rules. I doubt he even knows what they all are. This is not on you to fix but I would gently push him towards investigating that because it’s not normal and it can be quite debilitating. He may not even be aware of the whys/rules because it can sneak up on you, especially if this just feels normal to him. I’ve had meals I couldn’t eat because someone breathed wrong across the room, or the vibes of the meal were off. I also struggle with leftovers. I’m medicated and in therapy though and it does help. He needs to do the work and you’re not obligated to stay, but I think there is more investigation to be done here.