My grandsons Pumpkin is rotting

There've been lots of questions this summer about this happening on courgettes, squashes and cucumbers. I'm sure the bad weather affects it, as the flower starts rotting and it spreads to the fruit, so try removing the flower as soon as the fruit has formed properly. It's also extremely likely that the fruit haven't been properly pollinated as the poor weather stops bees and other insects visiting the flowers.
You could try hand pollinating with a feather or little brush, taking pollen from the male flowers (the ones without the baby pumpkin at the base) and transferring this to the female flowers.

I use butternut squashes ornamentally in my patio garden. They start to rot when they are due to, always at the base. I find I can delay this by standing them on some sand so that rain drains away better from them. Are your pumpkins rotting from underneath, or elsewhere about them?

I've put small ones in onion net bags and suspended them off the ground and that helped a bit. but with all this rain it is hardly surprising sadly :o(