He is a mole and he's living in my pumpkin hole

I'm guessing that Mr Moley has damaged the roots of the pumpkin(s) by tunnelling near or through them - causing actual physical damage or for the roots to dry out by having an air pocket next to them.

I also guess that remedying the damage will depend on the extent of the root damage.  I don't know much about moles but unless you can get rid of them from that particular area, I wonder how persistent they will be in those runs and therefore will continue to cause the damage?

I would therefore think that one option, albeit impractical and may cause too much root damage / disturbance in itself, is to replant them somewhere else or in a pot.

Would be interested in others' thoughts and suggestions though.

A relative swears by sprinkling used cat litter around which acts as a deterrant. This was a last resort as all other approaches (bar poisoning which he didn't want to do) had failed and having seen his garden now it appears to have worked really well. However whether it will deter quick enough to allow good growth in your patch is another matter and so Yorkie's suggestion probably offers the best way of saving your pumpkins.