Is it just staining? Could be from licking or something his skin is touching? (material, flooring, something in the dirt, etc)
Could you take photos of what you're referring to?
Jasmine - Female Miniature Poodle - born Aug 15, 2010
Loker Delgado Von Stalworth - Male GSD - born Jan 26, 2012
Koda & Zazu - 5 year old male cats
Alex - Male Cocker Spaniel (rescue) - RIP Cuddlebug 2007-2010