Is it possible that he has a sore throat? It was that that stopped Nougat from eating and she showed no other symptoms of a cold.

I've seen cats eat fairly well even when they've had bloody mouths (a few FIV cats before their dental surgeries) or bad teeth, and I remember my vet telling me that most cats will eat in spite of having terrible mouths.

Rutherford is a gorgeous hunk and it's distressing to think of him just not wanting to eat. I hope the tests show something, but nothing really bad, simply so that you can focus on a treatment.