What do I get my new boyfriend's dad for his birtday?

We started going out at the end of Jan but we're already VERY serious. I've been invited to his dad's birthday dinner in a restaurant with the whole family next week. What do I bring as a gift? I've been told that I don't need to bring anything but I really think I should. I don't really know his dad at all, I know that he likes Rugby and beer, and he makes granite and marble countertops for a living. Any ideas? Also, any clue what to wear?!!?