It's fine if you don't drink.
Accept a glass of something (wine, beer, or champagne usually) and leave it full in front of you.
If you drink your alcohol, someone around you will fill it up again (gesture of respect) so keep in mind that if you're not careful, you could end up sipping from a bottomless glass!
In Japan, there is something called yuino
(
http://www.google.com/search?hl=en&safe=off&q=yuino+wedding&aq=f&aqi=&...) - a gift-giving ceremony that is practiced between some Japanese families (old-fashioned, demanded by some, skipped by others).
I think bringing nice gift from you to the bride's mother is a good idea, especially if your son will be staying in Japan.
When they hit a rough spot in their relationship in the next few years, having her mother on your son's side can often save the marriage.
You should find out what her mother and father enjoy but I think fine crystal or Wedgewood are safe universal choices for this occasion.
Guests to hotel weddings take 30,000 yen (around 280US$) per person.
As the mother, you will not be expected to take a gift or money to the actual event.
You probably already know this but hotel weddings typically cost 30,000-50,000 yen per person to hold so even small weddings cost the equivalent of US$25-30,000 and upwards.
Sometimes it helps to know who is paying for what because those conversations are always being had in the background and often come back up later in life.
Any monetary gift you give the couple you should give directly to your son (in private when his wife is not around and strongly emphasize that it's an emergency fund).
Provide a separate non-monetary gift to the bride.
Obviously, none of this is set in stone! This is my advice on transversing some of the bad aspects of getting married in Japan... -_-;