Please be on the look out for an increase in his symptoms if he is going back on gluten. It is quite possible, since he did get some relief from symptoms, that he had both celiac and an inflamed appendix. Having an inflamed appendix wouldn't cause him to be 'borderline' positive on celiac testing.
I agree with ravenwoodglass. With both a family history of celiac and borderline blood tests, it's entirely possible that he had both acute appendicitis and he is somewhere on the celiac spectrum. I had never heard of "chronic appendicitis" so I looked in some medical textbooks and could not find it described. Is it some sort of "new" diagnosis which has not made it's way into the medical textbooks yet?