What can cause my 12 year old son to go so long without having a bowel movement?

Common problem: Various degrees of stool retention are common in childhood.It can be triggered simply if a kid decides not to poo when nature calls & then has a painful one the next day.Kids may begin retaining stools to avoid pain & gradually alter their intestine.At school age the kids want to avoid school bathrooms & continue the pbm. A complete eval by doc/xrays/6m+ of rx & it may improve.