When he's older, we'll tell DS that Santa embodies the spirit of giving and that we give to each other to keep that energy alive.
I like this. I may borrow it. My parents told me something similar, but focused more on how keeping the idea of Santa alive for my younger sibling/cousins was a gift that adults and older kids could give. I was very excited to be included -- but DD is likely to be the youngest cousin on DH's side and the only child on mine so I like your emphasis a lot.
We also plan to let DD lead the Santa thing. My family had Santa as part of the celebrations growing up but it wasn't heavily emphasized (and I always knew stockings came from my parents -- Santa did not give socks and PJs). It was a low-key Santa experience, though my brother and I had a lot of fun trying to catch him.
As for traditions, DD was born on Christmas Eve and I don't want her birthday to get lost among the holiday so I hope to do a little something special for her, just the three of us, each year on Christmas Eve before we join family for larger Christmas celebrations. I also would like to set up an advent calendar when she gets older with a mix of Christmas big and small activities (go ice skating, call grandma and sing a carol to her, etc) and chocolate. I figure this will make sure that we actually take the time to celebrate leading up to the holiday since it is a crazy time of year.
As a kid we often drove around on Christmas Eve, drank cocoa and looked at lights. I would like to do that too.
We alternate holidays with our families and may spend Christmas Day at any number of places so I'd like to carve out a few hours of family time each year on Christmas morning to open presents and have breakfast before joining the holiday chaos with the rest of the family. I have a small family and treasured the quiet times before the larger extended family joined us.