There is a section in the God's Love book in the chapter about holidays, not sure if he has a copy or if there is a way to obtain one, but you should look into it as it has an interesting paragraph about JW's with unbelieving mates. I'm not sure of the exact words it uses and I'm pretty sure it was applied to Thanksgiving (although I don't see why Christmas or any other holiday couldn't be used here) but it says something to the effect that if a JW has an unbelieving mate that wants to get together with his/her family for Thanksgiving, the JW might see it as simply an opportunity to get together with his mate's family for dinner and to enjoy each other's company. He doesn't have to see it as celebrating the holiday, just as a time to get together with family that he might not otherwise have a chance to see, since this is usually when most people have days off from work.
Use this as a starting point, just to establish that while, yes, your family is celebrating the holiday, like Christmas, for him it can simply be about the association, just getting together with your family and talking, laughing, sharing stories...just enjoying spending time together. He doesn't have to join in the festivities, like gift giving, singing, decorting a tree, or whatever the case may be, if his conscience is affected by it.