He's the same boy that he's always been diagnosis doesn't change that. He's always been autistic and had ADHD, anxiety is at the forefront of ASD.
There's nothing school can do now to help with his GCSE's like extra time etc. because concessions need to be sorted months in advance, so if you think it will be a shock for him, I'd not say anything as yet.
Did school initiate his diagnosis or have any input? It's usual for whoever does the diagnosis to inform your GP, school and the parent if under 16 or the young adult themselves if over 16. You may need to ask whoever did the assessment if they are sending a report etc. direct to your son.
Also if school have been notified, they may mention something to him. Ring the Consultant's secretary and find out who has already been notified, so you can let them know not to mention it until after his GCSEs.
I'm pretty sure he'll have noticed his differences and possibly looked into at least ADHD as his friends have likely ribbed him about it, so his diagnoses may not come as such a shock to him, hopefully he'll embrace it as it explains why he is a bit different.
When you do tell him, it's very important to stress, different does NOT equate to wrong. Please stress that as much as you can.
If all people are computers, the NT people's brains are wired like Windows, the ND (neurodiverse, covers a lot of diagnoses) people's brains are wired like Mac. But they are all computers and they can all contribute to society, they can get the same answers, but in a different way.
The Rebecca Burgess cartoon is helpful too, the-art-of-autism.com/understanding-the-spectrum-a-comic-strip-explanation/
You can't change the past, so concentrate on moving ahead, be as clued up as you can be on both dx and answer his questions as best you can. The dx will help in further ed and uni and the workplace, so it's not happened too late at all.