Annular pancreas is an abnormal rotation of the ventral pancreatic bud. It encircles the 2nd part of the duodenum like a ring and narrows the lumen of the duodenum. If the pancreatic ring is tight enough to narrow the lumen enough, it leads to vomiting. The bilious vomiting implies the obstruction is distal to the Ampulla of Vater.
Pyloric stenosis -> NONbilious projectile vomiting, olive-shaped mass in epigastric region
Esophageal atresia -> would be non bilious as well. usually neonates present with drooling, choke and vomit with FIRST feeding, this boy is 3 yrs old so it is unlikely.
Biliary atresia -> Fibro-obliterative destruction of extrahepatic bile duct, leads to cholestasis. Usually newborn with jaundice, dark urine and acholic stools because bile backed up.