This is a mystery isn't it? Poor boy. I hope his appetite returns soon.
It's good all tests are normal, but doesn't help the diagnosis. Could he have got into something which disagreed with him at all? Even though it didn't cause obvious GI symptoms?
Has he been through any kind of unusual stress recently? Family arguments, moving location, missing someone etc?
If not....then it is a mystery. I am guessing he still gets the same amount of exercise?
Another possible thing to try is a stool analysis test. That would pick up any possible GI inflammation, or parasites etc. Such things might not show up on blood work even though general systemic inflammation does show up in Serum C reactive protein and ESR (Erythrocytce sedimentation Rate.) So his blood test would show tings like that but possibly may not have markers for a mild GI inflammation
How are his ears? If something is wrong there it could cause a mild nausea perhaps. Get the vet to take a look and to check for nystagmus (unusual eye movements)
Is there any possibility he has any kind of pain anywhere? Pain can kill appetite. You could perhaps find out by gently feeling all over his body from head to toes, all over, and watch for his reaction in any certain place. A yelp is obvious pain. But even the slightest flinch could mean there was some discomfort.
It's amazing how many dogs keep going, keep playing, keep walking, even when they have some discomfort or pain somewhere.
Have you tried tempting him with some food? Something really yummy and tasty? A bit of plain grilled meat or fish perhaps? Or a sausage?
I hope you find the answer, and he's eating again soon.