Preparing a 9yr old for assessment

I’m guessing that your son is having the assessment because he is struggling in various ways at school or with life generally? 
I think the important thing to focus on when talking to him regarding the assessment is that he’s struggling and you want to do all you can to help make life easier for him. And that the best way to make life a bit easier for him is to better understand why he’s struggling with certain aspects of life, and that seeing an expert in why some children struggle with school etc is going to help you all find better ways to support him and help him to experience less stress and difficulty in his day to day life. Basically: “I know you find some things difficult and I want to do all I can to help you with that, and hopefully seeing this person is going to help us to help you.” 
In terms of Autism and directly addressing that: have you or has anybody ever mentioned this word to him in any context already? Does he know anyone at school who is autistic? What is his understanding of the word? What point are you starting from in terms of his understanding of autism - from nothing? Is there anyone he admires or likes that you know to be autistic - and potentially positive role models for him? There are quite a few actors, musicians, scientists, tv presenters etc who are autistic - and I think they can be helpful to talk about as they can show your son that you can still have a good life and be autistic. Chris Packham is a good example. 
Essentially it’s about trying to focus on all the positives you can, and also maintaining the focus on the point of all of this: which is to help your son have a happier and easier life by find out how best to support him. 
If it’s any comfort my youngest was diagnosed at 10 years old and he was fine about it - in fact I think it was a relief for all of us to finally understand why he was struggling so much and to have the school have to now acknowledge that he needed more help. It really is a good thing. Good luck! 

Hi Kate

Thank you for responding and we have been slowly introducing him to the idea without making it too overwhelming. He knows about the assessment now and really seemed to get it and understand that it is all to help him now and in the future. I think he has obviously been thinking about this a lot over the last few days and it has caught up with him now. He is still awake as I type this and in a complete meltdown, I ask if he wants to talk about it but he says that he doesn’t know how to explain how he feels. I totally understand that as I very often feel that way but cannot put my finger on why. 
Great tip on perhaps doing some research together and showing him that there are hundreds of neurodivergent people on tv that talk openly about it and advocate for the wider community. 

Very nervous times but hoping it all goes well for him and thank you again for your reply. 

Hi Kate

Thank you for responding and we have been slowly introducing him to the idea without making it too overwhelming. He knows about the assessment now and really seemed to get it and understand that it is all to help him now and in the future. I think he has obviously been thinking about this a lot over the last few days and it has caught up with him now. He is still awake as I type this and in a complete meltdown, I ask if he wants to talk about it but he says that he doesn’t know how to explain how he feels. I totally understand that as I very often feel that way but cannot put my finger on why. 
Great tip on perhaps doing some research together and showing him that there are hundreds of neurodivergent people on tv that talk openly about it and advocate for the wider community. 

Very nervous times but hoping it all goes well for him and thank you again for your reply.