I would say something like this "h, I am planning to have photos taken w/our son and Santa on xxx date. I thought it would be nice to use this photo on Christmas cards so that our family can see how much our son has grown. Would you like to be in the photo w/us?"

This leaves the option up to him.

Sit quietly, the answers will reveal themselves when you least expect them to.
The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.