I feel sorry for my little Grandson

Little you can do; he will gradually be terrified of making a mess of any description anywhere; exactly like my ex sister in law and her son. She spanked him with a hairbrush because he spilt washing powder over the kitchen floor. (Long time ago.)
I shuddered when I saw a magazine feature on a beautiful house furnished almost entirely in white; 'my sons have grown up in a white environment ever since they were born and they are trained not to make a mess', said the beautifully pristine chatelaine, grimly.

DD used to be quite obsessive over the state of their kitchen floor - not as compulsive as OP’s family, but it was always immaculate, despite the best efforts of her 3 children.
Then they got a puppy.......
?

Heartbreaking

Agree bluebell but I feel for him under that degree of oppressive control.

Sounds like the way to go crazyH don't fall out with your family over this fuseta or your grandson won't have anywhere to relax and enjoy himself.

Oh yes let him be as messy dirty and wet as he wants when he’s with you keep the water bombs for your house and his memories will always be the fun he had with you

It’s a real shame because he’s going to grow to have his parents ocd with cleanliness and tidiness but occasionally people do the exact opposite so maybe he ll grow up to be an untidy mucky Long haired teenager with a degree in woodland studies or how to naturally compost the world ?

The thing is crazyH ... it doesnt always work that way.
Some parents insist on their rules where ever,what ever, when ever.

Still, may not be that way for fuseta and her family

When he is at your house, he can do what he wants . Their house, their rules. Have him over at your place as often as you can, so he can have some fun time. Lockdown will soon be over ......

Perhaps, when lockdown is over (fingers crossed but not holding my breath), your grandson can play messily at your home.

I don't think there's much you can do about things in your daughter's home unless you seriously think he is ill-treated.

I wouldn't say anything or you may find access to your grandson restricted in the future.

See him

Goodness I so understand your perspective.
I'm sure you realise, but for your sake I feel obliged to point out, there is very little you can do about this.
You risk the parents wrath if you stand up (as they see it) for your gs against their choices.
Which could result in you not seeing him at all.
Been there done that.
Do you get the opportunity to see hom away from his home?
Can messy play be enjoyed by you both at your house or the park maybe?