From Travolta's Biographer

An explanation:

In private, [John] and Kelly were always very practical and positive about Jett’s health. They put him on a detoxification programme and encouraged his love of the outdoors, of sports, swimming, cycling and hiking.

In public, John clearly adored his son and spoke of him in glowing terms but he was always wary of delving too deeply into the details of his illness. It was simply too painful a subject.

Even when people saw Jett and it was obvious something was wrong, John refused to talk about it. In fact, the only time John made any public comment was when it was suggested that his son was autistic, an allegation he strongly denied and which hurt him deeply.