It can be argued that it is a question of seniority as a leader.
Obama was new when he bowed previously. As a world leader, he is now senior to the new king . It is the new king, the less senior that must bow.
It won’t happen though
Here's another way of looking at it. Saudi Arabia is the home of Islam. As the leader of that country, the Saudi king is, by title, the most senior of all.
Obama is only the ruler of a yet-to-be-claimed province of the caliphate. So I'm guessing that it is still Obama who must bow.
Perhaps John Kerry will weigh in on this matter and clear it up for us.