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The way you make a reader feel anything about a character is by how he acts. You can try telling the reader stuff about him that is contrary to how he acts, but it won't work. The reader will still judge him, will still feel interested in following him, or not, based on how he acts. 

So this makes what you are trying to do very difficult. You want your hero to spend the first two thirds of the book behaving not like a hero, and still have the reader think of him as a hero. That is going to be tough to do. 

The toughest part of this, though, is not going to be the character's introversion, or their nerdiness. It is going to be their indifference. Indifference to great events is not an appealing or even interesting characteristic in anybody, least of all a literary hero. 

But not every quest has to be **the** quest. Not every protagonist has to set out to save the world, even if they pivot to saving it in the end. But they do have to have some desire, some goal in mind, some wrong to right, some heart to change, or they are just not interesting. 

So, rather than thinking of your hero in such negative terms for the first two thirds, figure out what else it is that they care about, that they are working for, while the dragon is going about fricasseeing the countryside. But, since you are intending to pivot to him being the uber-hero at the end, figure out why the pursuit of his non-dragon related goal in the first two thirds of the book leads him inevitably to the pivot to dragon slaying at the end. 

A book, to remain interesting, has to have a trajectory, as sense that it is going somewhere in particular, not just meandering around. The end has to belong to the beginning and the middle to the beginning and the end. So in this case, you need to work back from the pivot point and figure out what was the quest before the pivot, and why can the consummation of that quest come only through the pivot. Because if it doesn't, the end won't belong to the beginning, or the middle to either, and the book will not work.