My DS is 15 and would behave exactly like the OP DSS if he was at a santa train event. Sullen, full of apathy, negative, miserable etc. and yes he would drain the "joy" out of the day.
DSD2 also aged 15 would be very similar.
DSS aged 14 would be the same as DS.
The difference is none of the above would want to come, they would make it quite clear that they would rather stay at home, see their friends etc.
None of the above have any SEN. I would say their reaction is a normal teen reaction.
DSD1 aged 17 would be just like OP DSS, would want to come, but then be as miserable as the others.
DSD1 has recently expressed a fear of "growing up", has been controlled by her parents arguing and fighting over her and her siblings. She is scared to implement a less formal contact routine as she has seen her parents going to court and continuously fighting over contact details. That has taught her that contact with Dad is something that involves conflict.
She manages to hold down a part time job, has a long term boyfriend and study for a couple of A levels, takes on a lot of responsibility for her youngest sibling at Mum's house and does not have SEN.
In my opinion she simply needs some guidance, some honesty and some freedom. She needs to learn that going out with Dad during the week isn't a "crime" and that santa type visits aren't the most enjoyable way to spend time with Dad. She needs gentle encouragement to do more age appropriate activities. Just as OP DSS needs to learn. It's the parents job to teach this.