Those suggesting he can take the baby, no chance. What, because that child is interesting because he/she isn’t a ‘bother’ like the autistic child that this cow hasn’t been interested in.
well, no, that's not what people are suggesting. What people are saying, in general, is that life is sometimes shit, especially when you have a child with complex needs and a baby. When you have a baby, let alone another child, sometimes you have to decline invitations. That is how life works out sometimes.
When you have another child with complex needs then you have to do the best you can. For OP in this case it might be possible of her to handle DS bedtime alone for one evening, and DH goes to celebrate his mum's birthday. The best way for OP to be able to do DS bedtime routine is for the baby not to be there, hence DH can take the baby.
If that would work.
And i echo pp: all these families making sure all celebrations are ok for everyone, how often do you take the entire family to soft play or the trampoline park?
Unfortunately as we move through life there are occasions where we have to say no to invitations, or decline politely, or have a workaround that involves a bit of planning and a bit of sacrifice so that one person can attend. It is a sign of being a grown-up how we handle this.
And if other people handle it badly (tantrums, emotional blackmail) then - well, we have to handle them with grace too. Or else life is just too much noise and effort.