A lot of dogs with liver shunt or MVD will rub their faces and heads a great deal - almost obsessively so your little one probably rubbed it raw or cut his face somehow while rubbing it. I'll explain it badly but here's what I understand about the condition: The liver isn't correctly cleaning the blood and the toxins from proteins go directly to the brain(rather than into the liver for proper cleaning/disposal) after eating and cause many odd/bad symptoms, including the head rubbing, lethargy, loss of appetite, nausea, vomiting, even diarrhea. Once the dog has fasted and the toxins are out of the system, the dog feels better, eats normally and is playful again but shortly the toxins are working on him again and all the symptoms start over.
My Tibbe always had normal liver tests on his routine blood work but because he was symptomatic, has been tested for MVD with a fasting Bile Acids test, which were high after eating, then a nuclear scintigraphy followed by an abdominal sonogram, and the two radiologists which did his tests and read them and examined him said he likely has Microvascular Dysplasia which often gives many of the same symptoms as liver shunt and is treated conservatively with special diet changes and sometimes medications. Tibbe has done pretty well and had but a few bad bouts of nausea/lethargy/itching since he was begun on his hepatic diet last summer. He's most a feisty toot once again and doing well. I hope your little one doesn't have a shunt that needs surgery but if so, it almost always to correct their symptoms, I understand.
Here's some information I was given when Tibbe was initially having his symptoms:
Portosystemic Shunts FAQ