Telling your child about their autism

Hey everyone 

I would very much appreciate your feedback on telling your child about their neurodivergence. My 9 year old son had his therapy session today and his therapist has said that we need to have that talk in preparation for his assessment which is about 5 months away. She said it is best not to leave it as he can at least have time to accept it instead of dealing with that and the assessment all at once. This is going to be the hardest conversation I have ever had and it’s making me so anxious. How do you tell your 9 year old little boy who you love dearly in a way that doesn’t appear negative. I want him to know in a way that we can celebrate his differences and look on it in a positive manner. 
Any suggestions would be greatly appreciated as I know some people on here would have been in the same position. 

Thank you in advance