@dadon Don’t go behind his back like this person is suggesting, that could make it more dangerous and alienate him. He will need you to be someone he can trust and turn to throughout this abuse.
I would tell him that you feel strongly that he deserves better and that you are here for him regardless. Don’t put your emotions into it, don’t say it makes you sad to see him hurt, just say as your parent I love you and this isn’t a loving relationship.
Tell him to keep a small overnight bag at your place so he can come there if he needs to. Give him a key and then let him settle into knowing he can trust you and see you and your home as a safe place.
As for Christmas, change your plans and perhaps have a distraction like a movie so you’re together but not interacting the whole time. Also, no alcohol or very limited drinks