since the episode, he has had NC with me or my other kids, but does have some contact with DH.
Does that mean that you have tried to reach out to him and he chooses not to respond?
How does your dh feel about it? Would he like your s26 at his party, or would he feel uneasy?
If your dh would like his son to be there, I think lever's idea is a good one.
As it is last minute, it's a question of what his reaction will be - and you might have to improvise if needed... .