Have never done the birthday surprise, so I can't comment on that.
You say you are going to do several character meals, so choose the characters your son likes the most so you won't have to spend a lot of time waiting on autographs in the parks.
After you receive your planning video, get a list of the rides, and maps of the parks, decide with your family what you might want to do.
Last year we went December 15 - 23. I noticed the parks seemed a lot more crowded than previous trips I took in June. Since you are staying on property, I would take advantage of Extra Magic Hours. We can always get a lot done in that hour before the park opens to the general public. Like everyone else says, make use of the fastpass system.
This past trip in December, we didn't get to do a lot of what we wanted because of the crowds, so prioritize and don't try to do everything.
It was very cold while we were there so keep checking the weather before your trip. DD walked around wrapped up in one of the throws they sell in the parks. Two things we enjoyed seeing this past Christmas season were to Osborne Lights (if you go to DHS) and Holiday Wishes fireworks in Magic Kingdom.
Have a great time planning!