Ds was late diagnosed last year at 14. Like your ds, he's what used to be called 'high functioning' - he's academically able; very well behaved in school. Like you, we have various family members who are undiagnosed, but I confident would 'hit the criteria' for diagnosis. For ds, and me as a parent, the diagnosis has been nothing but positive -although in real terms/support it hasn't made any difference - the school had already put systems in place to support him. But what it has done is allow him to very overtly and openly acknowledge that the stuff he finds hard (sensory issues; b&w thinking; rigidity in his thinking; deserve for clear rules and those to be kept to). And then that's allowed us to therefore be more honest about the challenges, when he does need to manage them, and how. So sensory stuff, both of us are now more alert to and intervene faster - to get him some space and time out - even if just escape to the loo for 10 mins, whatever. Whilst in theory, we could have done all that without a dx (& did, to an extent) the dx takes all the pressure off - it's easier to shrug and say 'ah, yes, you have ASD, you find that hard, so let's try xxx' - where before ds would beat himself up that he couldn't cope in crowds, or hates interactions in shops, now it's not him, it's the ASD. I don't know if I've explained it well, but I can say, for all of us, getting the dx was a huge relief, and the dx process itself was really positive, and quite eye-opening as we all began to realise how much we were managing already, and how effective some of our existing strategies were.
The other really positive thing for ds, is he is in a gang where we already knew there were a few ASD dx - so ds went out with his mates the night he got his dx and immediately told them all - at which point two other mates (who we didn't know about) also said they had ASD/ADHD diagnosis.
The openness within his group means that are very aware of their differences (to each other), so do a great job of pointing these out to each other (mostly in a supportive way! Sometimes less so!) and supporting each other: they are forgiving of x wanting to stay home, or y needing someone else to buy his can for him as he can't face an unknown shop assistant today... I know ds is very lucky there, but ime many ASD kids are quite good at finding each other and forming little tribes of understanding :-)