My girlfriend extended family made her cry like a girl.
because I'm a gaijin.
I will make a wild assupmtion and guess their more lenient on gaijin women.
Anyway i wouldnt worry, just conduct yourself as you would meeting anyone important for the first time and it should be okay, I wont lie and say theres no chance of them disliking you, ontop of a mothers ctiticism of her sons girlfriends Japan has an element of slight racial descrimination
BUT this is only a minority, especially for families of people who date outside of japan.
Bring gifts for his family, you dont need to break the bank but something nice will be greatly appriciated, perhaps if your having a meal with his family in their home offer to help prepare ir ot clean up, or you know, just general politeness one affords family of steady partners in the west.
be yourself, be polite, and relax, and chances are they will think of you as one of the family.
I was looking after my girlfriends mom when she was ill, while my girlfriend was neglecting her doing other things, and now she jokingly refers to me as "Nearly-son".
I found her family and Japan in general more then polite, you get the odd look from strangers as is the case, but it doesnt seem hard to get treated politely, and with just a small bit of effort you can ussually get family to like you like their own family.