I can't believe this thread - and some of the accusations being flung at the OP re: not being welcoming etc. If there is still any doubt regarding her intentions the bare facts are that her SS is continually invited to attend all sorts of family occasions - important ones which many people might feel hurt if they were left out of like birthdays etc - but won't attend, despite him being 18, if these events happen to clash with a "non contact" day. He is NOT being excluded from anything ! Conversely, he will always turn up when "the rota" demands, even if that means him being bored stiff - because, shock horror, life goes on and the rest of the family sometimes have no choice but to make arrangements during "contact time". Remember, he is welcome to drop by at any other time .... and yes, it's great that at 18 he still wants to see his dad (rather than more or less disappearing off the face of the earth like many young adults) but being so rigid about it all is doing no-one any favours and you end up with a situation like this where the OP's worried that any potential sulking might spoil what would be an otherwise magical experience for a very small child, and where, quite possibly, the 18 year old might feel very fed up.
Let me tell you about my experiences with Santa and the "big and small" issue. I've got an adult child, who, when they were around 18 came along with us and our youngest child to see Santa in his grotto (well, my oldest didn't see Santa but YKWIM). That's fine - anyone who's happy to enter into the spirit of it is very welcome to come along .... and my oldest was asking the youngest what they wanted for Xmas, pointing out the different animals as we queued and so on. All good. Another year my two stepkids happened to be here when we paid the annual Santa visit ... like the OP I wouldn't necessarily have chosen to inflict that upon them, but regrettably we are often told at the very last moment whether or not contact is going to happen and I refuse to sit here in limbo waiting to see if they turn up or not. Anyway, the older child was either 16 or 17 at the time and made it very clear they weren't happy at our plans - which would have taken all of 30 mins max. We had impressed upon them NOT to say anything about Santa not being real, which, thankfully, they didn't (I would have gone spare) but what they did do was spend the 20 mins we were queuing winding up the youngest along the lines that Santa wouldn't be coming as they'd been naughty ..... a one-off comment is fairly typical sibling banter perhaps, but this was constant and they kept on despite being told very firmly to stop. Result: one upset younger child who was only 6 or 7 at the time .... it was all so unnecessary and mean, and really took the shine off.
OP - if you really think that SS won't enjoy the occasion, then I'd get your DP to bung him £20 and drop him off at the nearest shops while you do the train thing. It's pointless buying an extra ticket if he gets nothing out of it. Obviously, if he was prepared to join in and make a bit of effort then all well and good but presumably you wouldn't have posted if you thought that was going to be the case ? I just wouldn't be happy taking the risk that the whole magic Santa thing might otherwise be spoiled if an older kid is sulking, or maybe making disparaging remarks. Why shouldn't your 4 year old properly enjoy a treat like this at Xmas after all ? .... it's hardly an outrageous idea and SS probably did similar when he was the same age.
Long term, this whole rigidity is plain ridiculous. I agree, it seems he's missing out on more family closeness and fun by doing what the calendar says rather than embracing occasions as they happen or being spontaneous sometimes. I have no idea if he may have Aspergers or not of course but this is a consideration his dad could perhaps be looking into so if applicable, different strategies might be used to help his son realise that verging from the "norm" every so often is nothing to be fearful of. Otherwise, if DP feels he's still being unduly influenced by his mother regarding when he sees his dad - after all, she may welcome a regular pattern so she knows exactly when she has the house to herself etc - he needs perhaps to be a bit more proactive on that front, by being more persistent and insistent over, for example, special family occasions. He could also impress upon SS that he'd like to take him out for a drink and pin him down over a date - even if it's not on the rota - rather than, for example, just shrugging if SS says no. And if he says no, challenge him - is he doing something else that night ?, does he want to do something different perhaps instead ? In other words, try to make SS see that he's allowed to organise himself and do what he wants (within reason) and that actually, that's one of the best bits about being an adult.
Not that he should "have" to do any of this of course. Most young adults are chomping at the bit long before 18 desperate to do their own thing - but for whatever reason (lack of confidence ? fear of upsetting mother ? Aspergers ?), he seems unable to do this for himself and unless he's given a helping hand this very silly situation where he turns up to what - for him - might be mind-numbing and "uncool", whilst at the same time missing out on all sorts of stuff he might really enjoy.
It must be very frustrating and acknowledging that, or even having a moan about it, does NOT mean that you don't care about him or don't want him there full stop. Do think about the shops suggestion - I know it's a bit of a bribe and shouldn't be necessary but as it seems he's coming over no matter what it might be a "solution" if you really think he's going to take the shine off the Santa thing because he hates it ..... can't think of many 18 year olds who wouldn't be happy to mooch round the shops for an hour or so with an unexpected bit of money in their pocket.