How to talk to 8 year old DS about upcoming assessment?

[FiveHoursSleep]

My two youngest DDs both have ASD but were assessed when they were 11 and 10. They were really interested in the process and the reasonings behind getting assessed and they both knew they were 'different' already.
DD2 ( now 13) was quite upset by the diagnosis, and finds it hard to accept that has ASD. She tries very hard to act as NT as possible and gets very upset if her ASD is mentioned.
DD3( 10) couldn't care less, she is proud of being different.
DS has an assessment in a few weeks and I'm trying to talk to him about what will happen and prepare him for the kind of things he might hear me say. He tries very hard to be social, but can not read social signals at all, has no personal space awareness and is in peoples face all the time, and the other kids in his school don't naturally include him. But he has no awareness that he is 'different' at all.
I don't want to make him self conscious about it, but how do I explain this to him? I think I need to say something, don't I?

With both my DDs, they were in the room while I was discussing them. As I said DD2 got quite upset, but DD3 wanted to join in!
We are going to the same hospital this time, but seeing a different Paediatrician.

So you wouldn't try and talk to him about it beforehand?
We don't have an extra adult! If I have to reschedule, there will be a much longer wait.
Maybe I can just plug him into a screen or something...

When we went in for a triage appointment at CAMHS I refused to take DS in with me as we stopped discussing his issues in front of him a few years ago. The triage person insisted that we all came in together so my OH and DS had to sit in the waiting room for an hour until DS was called. It was really inconvenient but better than DS sitting through it all. That was for an onward referral not for ASD but when we did have the social & communication assessment the paediatric route assessors were completely happy to have a separate interviews.

I've since been told by the post-diagnosis CAMHS staff that they were unaware that young children (less aware of their difficulties than older children) were sitting through these discussions and agreed that parents should ask for separate interviews if they thought it appropriate. At the time we were fairly desperate to access the service and didn't want to rock the boat, but knowing what I know now I would threaten to make a formal complaint if your assessor tries to insist that you have to talk in front of your child or forfeit your appointment.

Thanks tartan, that is helpful. I am going to ring them tomorrow and see what they can come up with for him to do while I'm talking.