The only thing you can do is listen to all he has said and acknowledge it.
Tell him you understand and respect his reasons for staying away and that you are genuinely sorry that mistakes you made in his upbringing were so severe that they have brought him to this state. Tell him you are sorry you didn;t realise this sooner, and that you love him.
Ask if he'd like to meet you one-to-one, on neutral territory, away from half siblings and your partner, so you can give him your undivided attention. Offer to book a weekend away for th epair of you, maybe to go walking somewhere, as walking and talking is very therapeutic.
Tell him you love him with all your heart and that at any point, night or day, over Christmas, if he changes his mind, he is welcome and if anyone makes him feel otherwise you will be on his side.
Tell him that you respect his choice to stay away but you would like to contribute towards it if he'll let you, as you want to make sure he is comfortable and happy and able to celebrate. Offer to send him money or a hamper. Ask if there are any warm clothes or home items he would like you to get him and if he;d like, just send links to the right items and an address for you to send to.
I think this sort of respectful, distant love-bombing is all you can do right now. I feel for you both.