My 7yrs son's lips suddenly darkens and cracked with his gums dark. what could be the cause ?

Many factors: The darkening of the lips and mucosa could be due to a genetic condition called peutz-jeghers. Which is accompanied by intestinal polyps. Another possibility is dermatitis which is a broad term that refers to skin inflammation due to any number of causes. If your son is a mouth breather dry lips can cause darkening and dry the gums as well. I would see your physician or dentist to check him.