I'm thinking two things will spur my MC to action, both of them other people. First he's now of an age where he will be expected to take on more responsibilities, only he's not very good at what's expected of him. As a result he is going to be confronted by his older sister.

Second, and less interestingly, he's going to meet and fall in love with someone.

Thinking on this I realize that my MC is very passive, which is one of the reasons he would make a very poor King. One of the major plotlines involves him, and his elder sister, working to change the Rules of Inheritance.