Ultraman

The answer to all your questions is -- ASK!
Ask: your bf's mom the purpose of dating your mom to have dinner.
Ask: is she (your bf's mom) expecting you to join the dinner.
Ask: all the people involved in your wedding to let you know what's going on for the things happening behind you, cuz YOU ARE THE BRIDE!
Ask: your parents for the amount of the lai gum they're expecting; and ask your bf to ask his parents for the amount of the lai gum they wish to give. Get a middle number that both of them would be accepted happily.

Things are not that worse actually, just need some time to get arranged/solved la. Good luck!

Ultraman

Maggie, not to convince you or make you feel even sadder...... The fact is, even true Caucasians (white people) would (or even like to) accept Chinese culture and give Lai Gum and dress in traditional Chinese costume la. What made your bf's mom proud of won't make her son's skin from yellow to white gua? He's still flowing Chinese blood inside la! And how come white people will respect and accept Chinese culture, but what make your bf so resists his own culture? If he can't (or doesn't want) to admit/accept that he is a Chinese, how can other people respect/accept him? How can he respect/accept other people? And that's why he can't (doesn't want to) respect/accept you la!
When you keep saying he has so many good things..... , but first of all, if he really loves you I don't see why he can't respect/accept your culture (and actually it is his own culture too!) lor?!