how to tell my child about their diagnosis

Hello, my 7yr  (rising 8) old son has just been diagnosed. Any advice on how to tell him? He also has developmental trauma and went through the care system before being placed with me. This makes it more complicated and meant it took longer to get a diagnosis as lots of his behaviour can also be put down to past trauma. I think its right he should know but also don't want him to think he's 'odd'. Any advice on what to say/books to read with him would be great.