How to explain autism diagnosis to a child

Tailor it to his own interests but for example you could say they are like computers. They work differently and different parts but do the same job in the end. Or to that affect. 

and the craziest of them all Leonardo da Vinci, my favourite. 

I would start from explaining Double Empathy problem as it is the bottom of all misunderstandings he'll have in his life. 

I would start by saying that he has something in common with: Albert Einstein, Isaac Newton, Charles Darwin, Mozart, Michelangelo and Picasso. They all exhibited autistic traits. Modern autistics include Elon Musk, Anthony Hopkins and Dan Ackroyd. Create a positive image of autism first of all, point out the useful traits that autistic people have, before describing the difficulties and limitations.