When I was a kid, we did caravan holidays - lots of places but mostly Southern France. My parents have their own van but the sites we stayed on had eurocamp mobile homes/tents.
We always loved it - good pool complexes/entertainment plus able to get out an explore the local area - I used to love going on the markets over there.
Due to covid and family stuff, this year will be my 9 year old son's first trip abroad. We are going to zell am see in Austria - from the looks of it, loads to do in the area, plus hotel has a pool/trampoline/table tennis/eve entertainment such as theme nights. Obviously can't say how it's gone yet but I think he will enjoy.
In this country, we have enjoyed things like Cornwall near a beach but again, able to explore. Plus the lakes is an absolute favourite (hence Austria lakes and mountains region this summer!).
I suppose maybe finding somewhere with things similar to what he enjoys at home so, pools if he likes swimming, local area to explore if he is happy to, varied/new foods if he is a fairly adventurous eater.
I hope you find somewhere and have a fab time when it comes round!