My 5 year old son is complaning of his rectum itching. What could it be?

I took him to the dr and they said to check at night time to see if I could see any pinworms. I didn't see anything. But I don't kno if they come out every night. I don't kno what else it would be. He usually complains at night when he's awake and in the morning when he wakes up. Please please someone help!