Well, first, you are absolutely right when you mention a possible major theme in this coming-of-age novel: the theme of the struggle to fit in. In fact, there are quite a few quotes that can serve as proof and be used as primary source quotes in an essay. Let's look at a few of them in turn.
First, we hear the first inkling of this struggle when Chan speaks of his "O-Ma," or mom at the move from L.A. to Minnesota.
I didn't stop to think that she was probably upset too. I mean, she was leaving all her friends, her church, and Kim's Green Extravaganza, our store.
This is an interesting quotation by Chan because it shows the struggle to fit in is UNIVERSAL and that it even affects his own mother.
In reality, Chan's struggle to fit in in Minnesota is really a reaction to how WELL he fit in, especially on the soccer team, in L.A. Look at what happens after his last win, ... as Chan lay on the field in happy exhaustion:
I would probably never feel as happy in my whole life as I did at that moment. ...
No one on the team will ever wear number leven It is always the numbe of our compadre Chan, claro?"
Oh, how different it is in Minnesota! One of their first interactions cements Chan's idea that fitting in will truly be a "struggle." When looking for an apartment, the manager throws an insult to Chan's father.
I can't understand a freakin word you said. ... And stay out, chinks!
Now we get into the action that Chan chooses in order to help him move in: the decision to play football (instead of his coveted soccer). It is "justifiable violence" ... and in those two words we have the perfect quote for your essay! Just look at Chan's reaction to seeing his new school in Minnesota:
We followed the trophy cases, which contained rows of gleaming football trophies and framed pictures of football teams, plaques in the shape of Minnesota.
This shows the focus of the school, and it must become Chan's focus if he desires to fit in among the students. There is no doubt that Chan must play football.
But probably the most perfect example is Chan's reaction to his own thoughts about the Father-Son dinner that is coming up when he finally DOES succeed in football in Minnesota. Chan first imagines a reaction from his dad: "Son, I'd be proud to. When is it?" But this is a facade. Chan knows his dad is disgusted with the American game of Football. Chan's struggle to fit in is reflected in his one-word response to his dad's typical reactions:
He's probably go around spouting his favorite anti-football sayings.
"Anyone can build muscle, but building brain is more difficult."
"Football is so popular in this country because it provides people with an excuse to drink beer."
"In Korea, grown men would not waste their time fighting each other over a tiny ball."
Each one of these quotations is the perfect fit for a primary source support for an essay on theme. Chan DOES struggle to fit in, and proving that is a perfect focus for your essay.