Santa brings the main gift (or a theme). Last year he got an Innotab and a couple of cartridges. This year he's getting a Lego table and a set of Legos. Everything else is from us.
I have a feeling DS' belief in Santa won't last much longer. He is very logical and I can already see cracks forming, so I'm not sure this question will ever come up.
If it came to it, though, I would probably tell him something about Santa collaborating with the parents on who gives what. "Well, I told Santa I already bought you x, y and z, so he was only left with a, b and c to choose from".