There are some medical things that can cause this. Has he ever had a complete blood panel workup - especially thyroid?
Also, has he been checked for encopresis? Pediatricians sometimes don't know much about it, but someone should go to bat for the boy to have a full health check that includes this on the list.
I agree with the other two, about encopresis and finding a medical cause for his constant hunger.
From your tone, it does sound like you aren't recognizing that this underweight pale child is actually in need of more nutrition, for whatever reason - not just trying to be irritating.
Also it sounds like it's possible he has unchecked type 1 diabetes.
Very good point by RockRose
"Type 2 diabetes
Symptoms include increased thirst, frequent urination, hunger, fatigue, and blurred vision. In some cases, there may be no symptoms.
Type 1 diabetes
Symptoms include increased thirst, frequent urination, hunger, fatigue, and blurred vision."
Hate to sound too prosaic, but he should also be checked for tapeworm.
This sounds like a medical issue, possibly serious. Please take it ultra-seriously.