I understand where he’s coming from - it must look lovely with the sun, blue sky, sea and pools etc. but the reality can be that it’s too hot, sunburn, mossie bites, itchy sand everywhere etc! Ask granny to stop showing him so many pictures, and get him a coconut - IMO they’re revolting and if he thinks the same he’ll realise photos tell a tiny part of the story. It all looks shiny and brilliant but might not be all that.
As others have said have a chat about earnings and perhaps look at average salaries for different roles - teachers, nurses, police force and compare to known ‘high earning’ roles - there’s more people who can’t afford these trips easily than can afford them.
If he’s academic look at which he could work towards as a career, and if he’s more vocational look at what electricians, plumbers, landscapers and mechanics etc can earn when they’re qualified. Help him to plan a pathway that will enable this kind of travel when he’s older and let him know it will be even MORE enjoyable when he’s older and he’ll more likely remember it better and not take it for granted.
I think it’s hard to fully grasp the ‘people worse off’ at his age, unless he’s really empathetic and that might negate how he’s feeling but I’d definitely mention it, just not go on and on in a ‘you don’t know how lucky you are’ way. I’m sure he knows he’s lucky in one respect, but feels envious in another. It’s quite a conflicting place for an 8 year old and his feelings are valid because he’s not got an adult understanding of the world.
Also, look together at how many people - young and old - are getting in to campervans. It’s the DREAM and life goal for a lot of people - follow some ‘Van lifers’ on social media to prove this and see if there’s places and activities in the UK that he’d like to experience.