Harsh punishments do not a strong parent make... Teaching boundaries is showing you care. Time to get curious, not furious.
Hey there OP, breath. I'm a parent group leader and I run parent groups. Parents who are struggling to manage behaviour attend my groups. I pass on tools that they report back do help alleviate home-life.
I will offer a few tips here. I'm no expert, they may or may not work.
Perhaps change Christmas around if you can. Spend time with your son who seemingly doesn't care about consequences and is making threats of his own - he is pushing you to see how far you will go. My advice: don't rise to the challenge.
Get your DH to take your youngest out on Christmas Day if you can, then serenely spend time doing something christmassy. Stay calm, that is key. Your are the parent, your oldest son is not your boss. Christmas presents can wait. If your son doesn't want to spend time with you, fine. Later, you and your DH switch places. If you can, re-order the timetable. Serve Christmas lunch later than usual. If you have family around, delay their giving gifts too. By all means, have them visit you if that's the plan. Talk to them first about your son, reiterate that you want to get to the reasons for his rudeness and aggression, not make it worse. If someone wants to get on board to help you talk to him, let them. Get them to ask your oldest why would he want to hurt his sibling by rubbishing the magic of Christmas. But don't shame or embarass him. Let him know that's pretty hurtful and vindictive, get them to ask is that the kind of person he is inside or wants to be, when he wasn't raised that way.
OP, you are mum. Don't let him bully you. Set boundaries, now is not too late. Not giving him his Christmas gifts is unreasonable and asking for revenge.
If you can't jiggle the Christmas Day timetable by switching childcare shifts with DH, then that's cool. Running Christmas differently would be enough to shake your eldest up, especially if he is one that likes to stampede-unwrap his gifts before breakfast for example. Just do something different, anything. But don't do it in a way that ruins the day.
Ultimately, have The Chat.
Key is, stay patient, don't lose your cool. Tell him that you feel that he is angry and that you want help, but you can't when he is misbehaving (insert what he is doing). Tell him you feel hurt when he is.... (insert misbehaviour), ask him could he help you by telling you how he feels, what does he need (these will be basic needs, safety, security, esteem, independence, self-realisation aims - not material). Think about his answers carefully, can you help him with anything? Support positive action only. Set goals. Meet halfway or compromise on things that you are not ready for - like more pocket money or staying out longer BUT he has to do his chores, use his manners, homework first and make you a cuppa. Be assertive. Outline a plan, set it out like an agreement. Be empathetic and be in charge, but ultimately he has to note your feelings, be responsible for his behaviour as it will do him no favours in the future. If he can't sit down for this chat, try again and again until he does. If he doesn't like the agreement, ask him why. Do not accept typical immature responses. Just adjourn and return to the drawing board another day.
If he has hundreds of gifts this Christmas, drip-feed them. But don't do it as a punsihment, work out a reward system - there are 12 days of Christmas afterall. If he doesn't like it, tough
Remind him you feel hurt when he is (insert misbehaviour here).
May i suggest reading - The Me Me Me Epidemic by Amy McCready, The Parenting Puzzle by Candida Hunt. Faber & Mazlish books are brilliant too, especially Sibling Rivalry.
Bits of.these books have positively helped a dozen parents with 10-year-olds that I have taught. You have to find what works for you.
Getting to the bottom of his aggressiveness will take time. You might want to talk to a teacher, enquire about your eldest's behaviour & performance.
OP, be strong: show assertiveness and teach boundaries - that shows you care ultimately.
I hope you have a nice Christmas anyway x