I used to have a major Jew thing (weird, I know...) and while I cannot speak for your bf, for me: despite the fact that physical appearance was indeed a major drawing factor, the attraction went much deeper than just physicality and instead was a deep attraction to the culture (everything; family, spirituality, history, food, etc).
Your boyfriend sounds like he probably didn't feel a strong connection to the close to non-existent culture of white people in the US. When you strongly connect with a different culture and finally feel "at home" after often many years of feeling you don't belong.... you tend to be very physically attracted to those people as a consequence.
Also realize, to some degree this means your bf has had to alter part of his "God-given" perception of self (someone born a white American) in a way that separates him from those cultural standards which he previously and discontentedly, defined himself by.
We very much so define who we are by who or what (white, a weeb, honest or dishonest, etc) **we are not** --- and your boyfriend does **not** feel a strong physical attraction to his own ethnicity. Therefore, by stating to you that he favors Asian women, it also means he no longer feels a strong connection or attraction to women of his own background.
During such a cultural rebirth one also has to confirm that he or she will even be received and accepted by the people within said culture. If not, the cultural orphan risks an even higher sense of lack in belonging than they did within their birth-given culture in the first place.
Again, I don't know your bf but assuming he's with you for more than sex (he is....) he likely views Asians and the culture with adoration, respect and as a whole as something he just cannot get enough of.
It def probably hurts his feelings quite a bit to be at the point where he cannot even bring up "Asian" things without bugging you because, 1) in his own mind he means the opposite of harm and 2) because it undoubtedly implies you view him as enough of an outsider to the culture such that he's in no position to speak of it.
You mentioned that Asian people enjoy reading lengthy Reddit posts here, right?!
Lame joke and excuse. Everything I write ends up a novel to read ;{.