I'm really sorry that happened, and know I would feel angry & embarrassed if put in that situation.
Try to focus on the mechanics of the decision making process. What was wrong was not that his idea might not have been good or bad, but that he made a joint decision unilaterally and put both of your S & your MiL in the middle. Emphasize, that you are willing to work on the arrangements and that there will need to be tinkering as you try things and see how they go. But, decisions have to be made jointly. And that you think it is best for S to not have surprise changes from plans if possible.
After you settle down a little, try a different approach. Get back in touch. Say that you were caught of guard & upset, but you've thought about his idea, and see some merit in it. That we need to make these decisions jointly not unilaterally, so how does he think we can do what's best given his idea & some of the concerns you've both raised.
Yes, you'll still be pissed, but you also want him to feel like he can bring ideas too you, get heard, get some affirmation, and that working with you will be an opportunity for collaboration not conflict. He will likely not pick this right away, and keep acting in conflict mode for a while. Just listen, and if you get pissed, nicely and thoughtfully say you need to think on it and get back to him. Thank him for bringing his ideas up and that you appreciate being able to work together for everyone's benefit. You can be pissed and rant here rather than respond to your H in the heat of the moment.
Eventually, by responding this way, he will almost certainly settle down and start following your lead. You may even find that when you get back to him, having show you would take his ideas and needs seriously, that he has done some thinking as well. He may surprise you by volunteering that he thought more about it and isn't as sure as he was that it was a good idea.
Bring the idea here and we can try to help see it from the outside and maybe give you a way to either compromise or at least turn the discussion into a more cooperative working things out rather than a power struggle.
The whole process really stinks. I always wonder why our WASs want to go through years or decades of this rather than investing themselves in working to see if we can salvage the M, but there you go.
Me: 50 W:43 S6, S3 M: 12 yrs. T: 17 M is bad & Not happy Bomb Mar '14 S 5 Feb '15 D Bomb 13 Apr '15 (but "no hurry") DB Coach May '15 Wants proceed on D Aug '15 Starting 1-on-1 negotiations Sept '15