DS8 feels hard done by as a male..

Sounds like a good moment to start opening a dialogue with him about why things are the way they are. Listen to his worries - kids are so sensitive to unfairness, and he sounds like a sensitive soul.

Would be great to get him involved in some boys only activities (off the top of my head maybe a sports team?) so that he can form good friendships and be in an environment where he can't compare his treatment to girls' treatment.

Is he aware of the history of how women have been treated? That might help him understand why they're now given more visible opportunities.

I think this is an important topic, and you sound like a very empathetic parent, OP. Undoubtedly, our society treats some girls very badly (sexualisation, exploitation) and single-sex spaces for girls still have their justification. But for many white, middle-class girls under the age of puberty, things have improved dramatically in the last few decades, and comments such as your cousins are maybe due to over-compensation for the blatant anti-girl sexism of the past.

But maybe there is a lack of positive male role models or appreciation of masculine qualities? Is there a case for reserving some single-sex spaces for boys? Possibly. Interested in what others think.

It's tricky, I think it can be hard to get a balance in honesty.

For example, my nephew's primary school had an amazing initiative a few years back to try and engage girls in tech, they had someone come and deliver some after-school 'lessons' and they all got some kit to keep (only raspberry pis, but still). It was great, and I see the logic, but it was a shame they weren't all offered the chance to participate- the school doesn't usually offer anything like this so it wasn't even like just let the girls have this one thing. It was hard to explain to an 8 year old why boys weren't allowed to partake and given the same opportinities. Surely it would be beneficial anyway to have a mixed group, as it would make working together 'normal', as it should be.

There are stereotypes about boys that seem to, in some cases, be self fulfilling prophecies as well. I think all that you can do is support his interest, listen to him when he speaks about what's on his mind, and perhaps find some men in fields of interest he has to help inspire him.

Girls can join Scouts but boys can't join Brownies
To be fair the Scout association needed to invite girls because the organisation would have collapsed without them.
Scouts isn't "cool" anymore and boys prefer spending time playing Minecraft on the computer or whatever instead these days. Brownies however has not suffered a similar collapse in popularity.

At age 8 it's sad he's feeling hard done by. Does anyone at home reflect these attitudes? Does he have a good father figure or sibling?

I think this is a good opportunity to talk about damaging stereotypes on both sides. It’s not fair to portray boys as noisy and messy but equally not fair to say girls are bossy or gossipy or that boys have to like camo and sports clothes and girls have to wear pink and like sparkles.

It’s understandable when we live in a society where people like your cousin make such odd remarks. I think I’d be teaching my sons that her comment is nothing to do with boys or girls but everything to do with elevating herself. That’s the society we live in, unfortunately.

My boys have noticed similar injustices towards boys over the years. They’re in their teens now and both have a mix of male and female friends. It’s good that your son notices injustices and stereotypes. Just listen to him and keep discussing it.

I think, even at 8, you can talk to him about how people stereotype boys and girls and react to what they think is happening based on those stereotypes rather than more objectively and how this isn't good for anyone. And maybe point out how girls are often denied opportunities for things he likes because of this (the reputation Guides used to have for avoiding a lot of the fun outdoorsy things boys in Scouts did, for example). If you can cite examples you could then talk about what his expectations are when he sees a girl do something compared with a boy.

Just point out some amazing male role models, from a variety of different fields, to him. What are his interests? Are there any role models there? It would be a good start.