Hey Ducky:-6
As Chezzie says and I agree... "Keep the magic going as long as you can."
I would tell your son that Santa is very busy and needs the parents to help him deliver the pressies. He hides the presents in certain stores ,and the parents can pick them up at the store and wrap them with their child's favorite wrapping paper.
Why tell the children that there is no Santa when they still believe

Quite a few of my 3rd graders (8 to 9 year olds) still believe in Santa and don't care that others do not believe.
I love Santa and still believe in the magic:-4