The main problem definitely isn't that he's introverted. It has to do more with the fact that he has no confidence and rightly so. While him being introverted does get in the way of him making friends, there are actual quantifiable issues that he has, and him being introverted is the last of those. He's actually right that if he was taller and maybe beefier people would think twice before messing with him. Additionally if he was "good looking" people would be acting nicer to him, there are psychological experiments that have been done regarding this - people are positively biased towards good looking people.
So while the author's doing a great job, I think attributing everything to him being introverted is wrong.