First of all, well done for getting the diagnosis after a long fight. Secondly you need hugs, chocolate and wine and a bit of time to come to terms with it. I am with you, my ds1 was diagnosed last week and I am still up and down, even though it was really only rubber stamping about what we already knew.
As Indigo said, nothing has changed, he's still the same boy he always was, the diagnosis is just a pathway to him getting all the help and support he needs. I do understand how difficult it is to get your head around though.
We aren't going to tell our ds just yet, but he is only 8 and we have discussed it with his Ed Psych and the Clinical Psych that did his assessment and we all agreed that he isn't ready yet.
The NAS website has a section of advice about how to tell children of different ages here.
The school should definitely be told asap, but be sure to let them know whether or not you have told him.
Is he getting any support in school? The SENCO should be your point of contact, but they should also bring in the Specialist School ASD Inclusion Team, who can work with both your ds and the school to make sure he is getting all the help he needs and can advise the school about things like, safe spaces to retreat to, having a 'safe adult' to go to when he has a problem and also making sure any sensory problems to do with the school environment have been taken into consideration. They can also do some ASD awareness training with his teachers.
Don't feel like you need to rush around telling everyone straight away (with perhaps the exception of the school, so support can start asap). Take a bit of time, allow yourself to process it all then do it bit by bit when you feel ready.