To be worried DS13 is upset to have lost the magic of Christmas?

It's just part of growing up. The focus on him is changing and he does not have the same magic .... it is normal.

Have said all of that, do keep an eye on your DS over the next few weeks and any signs of depression. For him, what he is feeling may not be the typical aspects of growing up but something more and whether it is seasonal or longer term, getting the right support for mental health problems early can make a real difference.

For my DS, part of the problem is social media too. Some of his friends are on holiday and spent Christmas Day at a waterpark, some have loads of cousins who they get on with really well, others have had huge amounts spent on their gifts. Their Christmases aren't any different to what they have done in previous years but it's only DS' first or second year getting live reports of it and, as we all know, it's hard not to compare and contrast.

I wonder if all the build up you describe is actually part of the problem - if you spend a whole month doing loads of amazing, fun activities, then isn't it somewhat inevitable that you feel a bit flat afterwards?

Him also saying it didn't feel as magical as normal - well, it won't. Christmas changes as we get older and what's amazing to a younger child isn't going to be amazing to a teenager, unfortunately.

It's been a very dreary dark couple of months and I am really feeling the lack of sunlight and vitamin d this year and I'm normally fine. I feel extra tired. Could it be this? Get him out in the daylight and get some vitamins into him. The days are getting longer now.

My DS is a similar age and, whilst not in tears, was a bit grumpy yesterday and it came down to the fact that Christmas just hadn't lived up to his expectations. There wasn't the same build up as there hadn't been any school events, we hadn't been to see Santa or even to do a light trail or anything; he'd asked for some quite expensive presents so there wasn't much budget for "surprises" and how could any surprise be as exciting as a slinky or a whopper cushion?; grandparents are getting older and are more set in their ways plus the dynamic is totally different with a teen than it is with a younger child; he is more dependent on his peer group and not really being in contact with them for a few days is odd.
I think it's all part of growing up. Christmas doesn't happen to you when you're older so you have to create it. I didn't do "I told you so" but did mention that perhaps next year we will do some pre-Christmas activities like the lights trail I offered to get tickets for about ten times, do something with family friends and their DC even though those DC may not be my DC's good friends any more like I suggested several times, make a gingerbread house again, like I suggested several times. We have made some plans for this week including watching Home Alone together, getting the Lego out and building a couple of sets, going for a walk with one of the aforementioned friends (who, interestingly, are having a similar experience with similar aged DC) and we might even make a gingerbread house.
I think DS is at a stage where he is pulling away from family and, for 50 weeks a year, it works for him to be centred on school, friends and sport but it's gone a bit wrong at this time of year when so many do revert to being family focussed. In a few years, when he is more independent, he will be able to do stuff with friends but whilst he and his mates are largely dependent on lifts and parents facilitating things, there isn't that option. It's actually been really nice to see him and his older sister spending more time together again as it's the best option available. It's the same when we're on holiday.

Probably just the January blues that most people get when the anti climax of Christmas is over.
I usually book a holiday to look fwd too, cheers me right up.

What was different this year? How was it so magical before and what has changed other than him growing up a bit? Do you still do the magical things you did in prior years or have you stepped it back and don’t bother so much now he’s older?

Ds and I still do the Panto together every year, still get the special treats we don’t have the rest of the year. We always do a jigsaw together as a family. Ds even said this year how he really enjoyed and was happy to still gave a Xmas stocking to open. He’s 25 now but we still do all the traditional things we always did.

Do you also feel the magic has gone? Can you make some new traditions together which can carry in even though he’s older now?

Surely it’s normal at that sort of age. IIRC I was about 14 when the fizzing magic and excitement of childhood Christmases disappeared. I was sad about it, but it certainly didn’t result in any sort of mental decline. And I still enjoyed Christmas.

I agree it could be hormones/puberty. He'll be ok and you can reassure him that christmas is still magical and exciting, maybe you could make a plan together of what you can do next year that is fun so he can look forward to that and making new traditions. In our family we have a whole list of things we do in December that we look forward to like going to visit the houses near us that have great lights, we take popcorn and hot chocolate and make a thing of it. It's all part of the excitement and tradition.

It’s really good he can be honest with you about how he feels - that’s a strong positive.

Keep an eye on him but it could just be the start of managing emotional changes/hormonal shifts as he goes through puberty.