he gets on at school just fine (bright little wotsit) so I would pursue getting any sort of official diagnosis as I don't feel it's necessary
OP, I was just thinking about my own situation and this thread came to mind, so I just wanted to add my personal experience in case it's at all useful...
I was diagnosed with ADHD quite recently (late 30s). I've always been bright and very academic, straight As at school, 1st class degree, good career, etc. Lots of people were really surprised when I explained I'd struggled all my life because on paper I look quite successful. But my CV obviously only shows the good bits. It doesn't show all the times I've come a hair's breadth from failing, all the hard work I've had to put in to learn social skills that everyone else seemed to just 'get', all the struggles I have doing really basic domestic things despite being quite bright in other areas, and most importantly, all the ways I've always known I was different but couldn't put my finger on exactly how or why. It's just something I've always known. It's been a bit shit having that hanging over me for the past 30-odd years so the diagnosis came as a massive relief, personally.
I've always addressed each of my issues (social, domestic, work, academic, etc) as a discrete problem which I've attempted to resolve, but I never realised they were all connected. The doctor said my intelligence seemed to have masked a lot of my issues as I learned a whole heap of coping strategies to cover up my difficulties. But it was still exhausting and it obviously affected my self-esteem as everything felt like such a struggle, especially when I looked at other people (who weren't as bright) inexplicably breezing through life.
In terms of sensory issues, one example from my adult life is that I would often avoid social situations that I knew I would find difficult, like busy pubs, as there's just too much going on to concentrate on one person talking. I had my hearing tested as I was 100% convinced I couldn't hear people in this environment, but my hearing was time. Which left me even more perplexed as to why I couldn't follow a basic conversation in busy settings and added to my lifelong theme of "WTF is wrong with me?"
The great thing about knowing my issues are caused by ADHD is partly that I have a better idea of how to solve each problem, and also that I can articulate them to others so they actually understand. So with busy pubs and noisy restaurants, before the diagnosis I would just make excuses not to go and would try to avoid socialising in those places, but I couldn't explain why so it just made me seem standoffish or unsociable (or just plain weird!). Whereas now when I get invited to those places, I can explain my sensory issues and suggest an alternative venue, or at the very least, people would understand why I either decline to go or why I'm not contributing much to the conversation.
Basically, having the diagnosis makes me seem less of an oddball to others which helps me socially, it's made me far less harsh on myself for all my shortcomings and I've become more realistic about my difficulties and more aware of how to overcome them. I'm also expending far less energy on (1) musing about what's wrong with me and (2) trying to be like everyone else.
Obviously everyone's different and I'm sure loads of people get through life fine without a diagnosis (several of my older adult family members who quite clearly have ADHD seem quite happy - they're just viewed by others as completely crackers eccentric), but I just wanted to offer my perspective.