Talking to 4.5yo about ASD assessment

My 4.5yo boy knows he's different - I've told him his memory is special, and he knows that his behavioural issues aren't typical, and that his school experience (1h/day, 2 staff assigned to him) is atypical also.

We're hoping to have our private assessment in the next month.

He needs to know what's happening in advance. What do I tell him about the assessment? We're told you get a result on the day, and because of his twin it may be a one parent event, so there's a chance he would be present during receiving a diagnosis (or not, as the case may be). Should I work to avoid that? We don't normally offer iPad and headphones, but would that be worth doing?

What do I tell him about the process? What do I tell him if he is diagnosed, or isn't, or it's unclear?