We've done Centre Parcs twice with DD (once just as we were seeking DX, second time 2 years later and just 6 months after DX) - Asp and ADHD (reasonably high functioning - but not without her issues!!). It worked great as we could cook what we wanted and there were plenty of different options (many very familiar) in the restaurants. Lots of activities (admittedly lots cost money).
Yes sand on the beach of the lake, but lots of forest walks, cycling, little supermarket etc. Loads of nature to be seen too (rabbits, squirrels, birds etc), and fish in the lake. Pool with no sand but lots of different parts indoors and out, slides etc. Familiar channels on tv. If you bring your own bikes, ALL that 2nd para are free.
And because we drive there (ferry and car), we can bring comforts from home too (familiar foods, but also familiar pillow, teddy, books, and plenty of games and colouring things and other distractions we know she likes). And we bring lots of her favourite films on dvd for the car and in the villa.
We don't tend to do beach hols anyway. This year we're going to Disney Orlando, which will be interesting. But we've done Boston/Cape Cod (only 1 day near beach and that was because we were going whale watching on a boat), which was ok (but stayed in hotels which was a mistake - just about doable, but apartment/villa much better options, especially for space and also cooking facilities).
We've generally done self catering hols, not with a "beach destination" (although we've used my parent's holiday house in west of Ireland, but surfing sometimes among other tourist stuff, don't get beach weather there). They tend to work well.
And I'd agree with printing off pictures in advance - general holiday pictures, and also of where you are going (accomodation, local sights, beach, forest, etc). Talk about it beforehand, and about it being ok not to like everything but it's good to try new things because you might like some of them. Reassurance that you will find some foods he likes (bring favourite snacks in luggage maybe, just in case), but not everything may be the same. And I am learning very much that it's fear of the unknown and lack of routine that can be a killer here - so let him know what will be the same, or that you don't know what will always happen when but that some things you might do are (go to beach, go for walk in forest, ride a pony, have coffee and buns with mum and dad) and that other things you do everyday you'll still do (eat breakfast, have dinner, have a shower, go to bed and have a story...).