Occasionally my son's penis retracts into the surrounding skin. he's circumcised, so should this be happening? shouldn't the tip always be visible?

Yes: tip ( glans ) should be visible but some times will have excessive skin.or too much foreskin is left behind . alert In his next pediatrician visit , will explain the problem.

Fat pad: This is not unusual for smaller kids. They have a little "fat pad" which also can reduce the appearance of the size of the penis. Have his pediatrician check it at his next visit!