I don't really know your daughter's boyfriend, so I won't pretend to know exactly what to get him for Christmas. But... I do have a few ideas for how you could find out:
Buy a joint present with your daughter. You could go halves on it...
Seriously though, this could equal some good bonding time, as well as a chance to find out what their relationship is like.
It's a good way to show you care about their happiness, without having to snoop too much.
Check his Facebook. I'm pretty sure most moms check out their daughter's suitors on social media - don't pretend you don't! And Facebook (or Twitter or whatever else he uses) could be a good way to find out what his interests are.
If he doesn't mention them in his "about" section, you might discover his tastes based on his recent status updates or pages that he likes.
HOWEVER, if he ever asks how you knew what to get him, don't mention spying on his Facebook.