Poor little lad! Let him enjoy lots of untidy, messy play when he's with you, you'll both have so much fun. Baking, painting, making things with clay, sticking and pasting is all good messy play. And of course jumping in puddles on a wet day! I wouldn't worry too much about his clothes, dirt washes out! But you could keep a few spare clothes or a pinny for him. Could you perhaps keep a pet at your house for him to interact with - rabbits, guinea pig, hamster, cat, white rats (they're very intelligent and friendly)?
I had a friend like your DIL. Her house was clinically clean, every surface was wiped with bleach twice a day you could smell it when you walked through the door! You would never have thought she had any children, there was never a toy in sight, nor a child. Her kids got every stomach bug or ailment that was going round and were off school more than they were there. Mine on the other hand were as tough as old boots brought up in a home with 2 big dogs and a Mum who'd rather play with them than do housework. Yes they caught measles, mumps and chickenpox but they'd had their vaccinations and it was no big deal. Now grown with their own families they are equally relaxed with their own children.
One of my DILs is a bit obsessive about hygiene but she's gradually relaxing a bit. I think she's realising keeping two under two's from getting grubby is nigh on impossible. But her over-concern when the children fall/bump/scratch or graze themselves (however minor) worries me a bit. She makes such a big fuss about it
- antiseptic, plasters, constantly checking them, and of course they respond accordingly: lots of tears and clinging to mum. I always treated everyday tumbles and 'hurts' with a hug or cuddle and a 'There, all better now" and off they went to carry on playing. Perhaps I was too casual (but not uncaring), but they survived!
Enjoy your time with your grandson, in time his parents may come to realise their standards are a bit OTT.