My brother tested positive on a celiac panel. Does that mean that he has celiac disease?

Celiac diagnosis. The "gold standard" procedure for diagnosis of celiac disease is the upper GI biopsy, but a skin biopsy for dermatitis herpetiformis (itchy blisters) from an elbow or shin can also be definitive. Both biopsies show IgA antibodies in the tissues. Many people without "perfect" diagnoses still find they benefit from gluten free diets. Your brother's blood test is suggestive but not definitive.