Grandson Age 7 Was Born With Small Area Above Rectum

Grandson age 7 was born with small area above rectum that was not opened and he is always
pulling at his underpants and cloths .have checked no rash or redness but he is always pulling at his
pants he wears boys boxers for age 6 to 8 so we know they are not to small or tight. Have asked him over about any one touching his private area he says no. please advise what I need to look for to help him. He is not in pain just always pulling his underwaer