If it turns out to be that,
surgery will be needed to remove the appendix. If
appendicitis is ruled out, then it could be any number of things. But please do get him checked immediately if he hasn't been already. An appendix that is infected can rupture, or break open, into the abdominal cavity - which is a serious situation (I know, because it happened to me when I was 15 years old).
Hope this helps!