BlueSkies1

A lot of eggshells you're trying to not tread on and break, amy. I can see why, but I also wonder.
If your son says that people are allowed to bring SO's, it might clear it up for everybody...and then...

Your friends can just show up with their son. Some consider that a faux pas. In another way, it's simply a way to act without a discussion. It's not like they would be sent home, I imagine.
Bottom line--does your son love these people and would want them there on his special day?
I would let that be the deciding factor. If one child with autism throws someone into a hissy fit, that person has a problem.
Life is about sharing and loving, and definitely about accepting into our world people who have special conditions, and the accomodations that they and their immediate family needs.

It's just what I would do if I was them, and I thought he loved us as a couple enough to see that we had to bring our son along. One might think, that since he knows this family well and their son's needs, that it is to be expected for him to be where they are.