Hi there knotguilty, I'm sorry you and your son are going through this. First off, crohns can only be confirmed through a colonoscopy with biopsy. There are several types of inflammation you can get that are not crohns but given the location and description, crohns would be a possibility. If it is crohns it is by nature chronic but there are all different manifestations of the disease, some of which are very mild but it is impossible to predict. Some are ill frequently while others have years to decades of remission in between flare ups. The only thing you can do is get good medical treatment and proper medication for it and make sure you educate yourself with good sources and sites like CCFA.org. There are a lot of scams and miss information out there that claim cures through "natural" products and techniques- just a fore warning, No matter how good they sound they are misleading, false and prevent proper medical treatment.
Irritable bowel syndrome will not show up on a CT scan. It is a functional condition, no inflammation, no damage is done and is a diagnosis only given to those who have symptoms but everything else has been ruled out. I hope the GI appointment gives you some answers. Let us know how it goes!