Google is your friend.

I actually loved the movie but one of the things the writers got wrong, and they frequently do this, was how they refereed to King Leopold II of Belgium. I am a royal historian and it does bother me when movies get styles and titles wrong. It is so easy to just look up this information so there is no need to get it wrong.

At first they called him His Serene Highness. That is one of the lowest styles for a member of royalty. The correct style for a King is His Majesty. They called him His Serene Highness twice in the beginning then at the end they called him His Serene Majesty (a style that doesn't even exist) but in the end they got it correct and called him His Majesty. Again, no reason to get this wrong.