They have known your stepson since he was 10. They have seen him grow up. You are yourself a stepchild to your stepdad.
This is a watershed moment, in my opinion. You need to lay it out. Does your stepson know yet that he hasn't been invited? The whole thing is predicated on the idea that by 17, people don't want to join in on family holidays - it's effectively parents and kids, so boring for young adults. I get that. But it feels bad because he's the only one excluded, and because no choice was offered.
If your fiancé won't go, how on earth can you go? Think about it. How would you feel? The message would be bad for everyone. And the most vulnerable person is your stepson, and the second is your fiancé.
We are always dependant on the fairness and decency, and kindness, of others when we join a family as step-members. The acceptable and love should, ideally, be quite unconditional. Anything else doesn't work.
I would go to your mother, and explain that your stepson and fiance are very upset. I would suggest that she and/or your stepdad speak directly to your stepson and tell him they're sorry, they thought he wouldn't want to come as it would be boring for him - but, OF COURSE, if he'd like to, he's more than welcome. They need above all to make him feel valued. And only they can do it. (or your mum can talk to him and explain that your stepdad is xyz, but it would be better if the stepdad was also involved).
You need to explain to your mum that you guys are a unit. And it needs to be very clear that if this isn't quickly and carefully resolved, then it will indeed cause a rift and you and your dd won't go either, because - and I'm reminded of 'King Lear' Act 1 here, and Cordelia's dilemma - (!!! sorry - yes ok all attack me for that!!) - you can't just love your parents. You have to also love your fiancé, and his child. You love your parents no less and no more than they are due. And the same for your fiancé. So they can't expect you to denigrate him like this and make his son feel unvalued.
Because then they force you to choose.
Which is not a good idea on their part. For anyone.
Have the conversation with your mum.