What Causes Loose Stools Along With Bad Odour In A Child?

Hello..I have a question my sons beeps smell really bad and is complaining of watery stool and cramping in stomach along with his stomach alittle hard and a little bulging of the stomach,makes him very uncomfortable and restless,crying its happened once or twice before maybe once a yr is this common and what can I do about it he s 9yrs old sincerely concerned parent....