All of the thoughts mentioned here did run thru my mind.
This is a tough one!
First, it is the holidays, so you definitely do not want to think about saying anything right now. You have time to think about this.
While I would never want to invite 3 members of a close family who have been close friends/acquaintances for so many years.... One can't make that assumption about others. While the mother of the bride might be savvy enough to know that 'family is family' in some situations like this, that might not be something that is so strong in the mind of the young daughter who is getting married???
Also, I think I would have to assume that Save The Date cards are being sent to all who are intended to be invited. So, I am not thinking that waiting for the invitation would bring any change. I am thinking that there is a name and address list, and all on the list who are being invited would be sent the card.
After reading thru the thread, and not coming to a strong conclusion as to whether I would, or whether you should, actually mention it, I went back and re-read the original post. I often find that this is very helpful in getting some insight. This is what I really noticed :
The other day, we got a "save-the-date" card for her wedding. It was addressed to DH, me and DS18. Our DS21 was not listed. DS21 has Asperger's Syndrome, so he is far less social than his younger brother. He has never been to a wedding and is curious about what it's like to attend one. He has asked me a few times if I thought we would be invited to the neighbor's wedding. I told him we would have to wait for the invitation, but I thought we probably would be invited.
We do not know your son. We do not know the degree of the ASD/Asperger's. But, it seems to me that he has more of an interest in attending a social event like a wedding than he does in actually seeing this particular young woman get married. I am not sure I would want to mention the comments you made in the above posts.. "he has never been to a wedding, etc..."
We also cannot really know how close you feel that your two families are. I think that I felt that we were that close... almost like family... then that might sway me towards mentioning an awkward subject.