I'm sure that your ex and his wife will understand how you feel about this, and if you get on with them (which sounds like you do) talking openly about it with them might help.
I think you should let the child go. I know it will be a wrench, but there will be lots of people to make the trip special, Christmas is special already and there will be lots of people to make a fuss over him.
Of course he will miss you, but he's old enough to rationalise that, and if as you say his dad and the wife are great with him, you know he will be safe and looked after.
Your DS will appreciate you trusting HIM enough to let him go, and he will be able to contact you when he's over here.
You have a good deal of time to sort out lots of things for you to do while he's away to keep yourself busy.
My DS is a little older than yours, it's always been him and me against the world, his dad lives few hours flight away and is grade A useless. In fact being useless is the only thing he's consistently able to be successful at. there is no way on earth I would allow DS to fly to him or spend time abroad with him, BUT that is because Ex is only able to think of himself, and has never ever put his DS first, even when given all the info and hints in the world to be able to do so.
(((hug))) i know this must be tough, but I think you ought to let him go.