His hat? Any wealthy Tudor man may have worn his hat.
It's not the jewels either. Though there are lots of them in this painting, they suggest that this man is very rich. They do not tell us that Henry was a king.
The correct answer is nothing. There are no real clues to tell us that the man in the painting was a king. Royal portraits usually had symbols to show who the person was and how powerful they were, e.g. crowns. Holbein did not use such symbols in this portrait. Henry is wearing a special garter (the Order of the Garter) but this only tells us that he was high-ranking nobility or a prince.