Wow that's frustrating...So he just won't eat anything on his own, only formula, and really has no feeding-response but just swallows the formula, or he's just tube-fed...Everything else it totally fine...And he's maintaining his weight on just the formula and IV/sub-cutaneous fluids? It sounds like your vet has covered all the bases...Has he mentioned anything about doing an Endoscopy to take a look in his Crop and GI Tract? Sometimes they will see something very obvious during an Endoscopy that you can't see at all on plain-film x-rays or Barium Studies, such as non-metal foreign bodies stuck in the Crop, the Esophagus, Stomach, etc...
Did he do a really thorough examination of the inside of his mouth/throat? How about his beak, did he see if Goofy has any sensitivity when his beak is touched/pressure is applied? I'm just thinking of things I've seen cause anorexia in birds without any other signs or symptoms, and a lot of the time it's either due to pain in their mouth/tongue/throat or from their beaks, or from a very small foreign body stuck in the Crop/Throat/Stomach, such as a wood-splinter from a toy, little pieces of plastic, etc. I've seen Endoscopy done where they also do a Crop-flush with the scope, and tons of little "bits" of things come out, usually little pieces of plastic, fabric, thread, and wood splinters...
Depression is always a possibility, but it usually doesn't happen so acutely and without reason, and it usually presents with other behavioral and physical changes along with anorexia...Usually with full-on anorexia that is so sudden and acute, it's due to an equally sudden and acute problem, such as pain caused be either an injury or from them eating/swallowing something they shouldn't have that is suddenly causing them pain WHEN they eat and ONLY WHEN they eat...Beak, Mouth, Tongue, Throat/Esophagus, Crop, Stomach...