Incredibly strange of you to go about this the way that you are choosing to. Incredibly strange. And the reasons you've given as to why, such as
"It's none of his business, he doesn't tell us anything about his plans."
You're the adult, try to remember that.
Secondly, you failed to mention originally that you do not own your house nor is it 'on the market' - again, strange that you're wording things the way you are.
Your house is council/Housing association so, why would you not just say
that you're looking at an option of a mutual exchange and not sure if you want your eldest child to know yet, incase of the possibility that it doesn't happen.
We used to be HA some years ago, before we gave it up and bought a house. But during that time, we viewed multiple places for a mutual exchange and they never worked out. Every time I communicated the possibility of it with my son who was then between the ages of 5-7. It's just healthy discussion.
When he was 7 he was well aware that we were looking at buying a property 3 hours north of where we lived at the time.
We explained the reasons, pros and process of the move, viewing, buying stages and kept him in the loop.
Again, you seem to lack something with communication. By changing the details of the story, you've come across far worse.
A mutual exchange viewing is far less intense in the process than that of selling a house and starting the viewing process with a house 'on the market.'
Sit your son down and say, there's a small chance that a lady with a house state where may want to do a mutual exchange with your home and hers and you're considering it as it's bigger. List other pros. And that you're having her over to view on Saturday. Could he take himself off out for a couple of hours between x time and x time while she's over.
Explain that she's viewing yours first and if she's interested, you'll all be viewing hers to see if you like the house and area and explain things will potentially move forward with paperwork from there. But thats only if both households agree and dont back out.
Sharing that info with him is no biggie.
You shouldn't be keeping this information from him, just be honest and communicate clearly. Have that respect for him and teach him to communicate.