I would feed him and help him get back to sleep. It could be a growth spurt (very common at 6 months). It could be that he's still a little "off" from all the holiday excitement (I know my kids are). He could be not feeling quite right. I really could be so many things.
He needs you. He needs you in the middle of the night, and it sucks because you need your sleep too, but he needs you to be there for him. It won't last forever. He'll get back into his routine soon. For now, just be there for him.
Please keep in mind that there might not be anything you can "do," exactly, other than be patient and respond to his needs. Many of us had kids who didn't sleep through the night consistently until 18 months! We didn't necessarily do anything wrong. It's just the way it goes.
Hang in there!