Nathaniel Asselin was a handsome, athletic kid who excelled in school and had lots of friends. But when he turned 11, something happened that would change — and ultimately end — his life. A therapist finally put a name to his symptoms: body dysmorphic disorder (BDD), an extreme form of obsessive compulsive behavior. On Thursday, Denis Asselin will end a 525-mile journey at Christopher Columbus Waterfront Park to increase awareness about the disease that robbed him of his son.