DS is 14 and has HFASD, assessment almost complete but he knows anyway. He asks if he is and says it would make sense. They know they are different to other children, having a name for it helps.
DS uses earplugs or big noise cancelling cans at school so distractions don't overwhelm, quiet fidgets (so classmates aren't distracted). Look at Simple Dimples things like that. Ask school to provide a safe space if they don't already have one, and a box of interesting stuff like kids magazines or something about their particular interest (I won't say obsession). School shouldn't say no to any of this stuff, best practice is to put things in place to support while diagnosis ongoing - if not take it to governors (there will be a SENCO one). We moved schools because the first rejected requests to do these things - you have to be your DCs best supporter and advocate and sometimes you have to put on your big girl pants and fight for them. Any talk at school about your DC just 'learning more resilience' is bollocks, you can't teach ASD away.
We don't talk about it much but when DS asks we are very matter of fact, emotional talk is useless anyway. Tell them directly that they can talk to you about anything they like. Be unshockable. Make home a sanctuary where they can be themselves without judgement. Outbursts are a way of coping, not an argument, we learned the hard way that scoldings escalate things massively, so when they are overloaded to the point of meltdown just be there and don't take it personally.
Don't talk about ASD as a negative thing, get upset, wish they were different or wish that it wasn't there, its important they know you don't wish away a part of them that they cannot change. One of DS's friends mother has done this with her ASD DD and the poor child is a mess.
Celebrate their interests and embrace it, lean into it and enjoy all their positive traits and qualities. Don't push them into interests or hobbies to fit with friends or the rest of the herd, it won't work but will make them feel shit about themselves.
Also what has helped with DS is kind of normalising it, telling him everyone is different, unique, the thoughts he has about fitting in are the same for everyone (NT or ASD). He just has a harder time with some stuff, he doesn't think of it as a big deal or some horrific thing because we don't make it one. That doesn't mean we are dismissive at all, quite the opposite but we deal with his needs in a practical way and don't make a fuss. DS knows he's different but it needn't be 'othering'. When they ask if they are like you (or like DH) say yes, because its true. ASD is what he is but its not who he is. He's 14 and likes himself the way he is because we like it, we don't reject a single bit of him and he sees that.