I have not been in your exact position, but similar enough to understand. In 2017 DD1 was 16 and in the grip of anorexia that very nearly killed her. She was hospitalised and tube fed in the October. When she was at her lowest point in hospital what gave her the will to keep going was planning Christmas that year. We bought Christmas magazines and looked up crafts online from hospital. We planned to have fireworks on NYE if she was home by then and Chinese lanterns (both will horrify most Mumsnetters but needs must).
BUT you need to do all this hand in hand with some really good professional mental health support. And not build one day up so much that everything falls flat afterwards.
You could make the cake and pudding and mincemeat soon.
I would concentrate on doing things together. Perhaps get him to pick 24 activities and write them on paper and you put them in a home made advent calendar. You pick which goes where. Just things like making popcorn, playing a favorite board game, watching a favourite film.
We made a wreath that year (and have continued to do so since) with ivy and holly and greenery from our garden.
Roasting chestnuts.
Make and write Christmas cards even if you don’t normally.
Make paper chains.
Get him to do a Christmas play list.
Support a local food bank and get him to choose the food.
Go to a Carol concert.
Walk around your neighbourhood if it’s safe and see the lights in the evenings.
Do him an old fashioned stocking for him with tightly wrapped treats in it (reslly novelty items; look on Etsy and eBay, not just token things like pants and soap to fill it up).
A Christmas jigsaw
National trust houses often have lovely Christmas events.
An Advent candle.
Do not rule out a steam train or panto. If he likes Christmas he probably likes the childish aspects of it.
Really importantly I would ensure you have a plan for when it’s over. Maybe do 12 days of Christmas so you have a gentle come down after. Nice food. Walks. Together time. Make some plans for the next Christmas. Write a list of what the best bits were and promise yourselves you will repeat them.
I really feel for you OP and your DS. My DD is well now and has graduated and started teacher training. But there were times in 2017 we honestly thought she was going to die. Sending you love and strength.