Only the sovereign is currently accorded the style of His/Her Majesty.

Only the children and grandchildren of the sovereign are accorded the style of His/Her Royal Highness.

The spouse of the sovereign (if it is a lady) is styled HM, and is accorded the dignity and style of queen, and retains that title (but not the throne) should her husband pre-decease her. If the spouse of the sovereign is a gentleman (as in the present case) then he is accorded the style of His Royal Highness and accorded the dignity and style of prince. In the United Kingdom the husband of a queen in her own right (for example HM Queen Elizabeth II) is no longer styled as "King Consort", nor is he accorded the style of "His Majesty". Instead he is regarded as a Royal Prince with precedence immediately after the sovereign, but without right to ascend the throne should the sovereign pre-decease him. As with all other royal princes he is accorded the style of HRH.

In addition to being princes and princesses the children of the sovereign are generally given subsidiary titles, as is the prince consort of a regnant queen (hence HRH Prince Phillip, Duke of Edinburgh).

This really can seem far more complicated than it actually is, and the easy thing to remember is that only the sovereign (king or queen) is His/Her Majesty. Everyone else is "merely" His/Her Royal Highness.