Puppy pottys on grass but does not potty on wet grass

I have a 3 month old puppy, it's the summer months here and I've trained him to potty outside on the grass. He does not potty or poo anywhere else now.

This night it rained and I had trouble making him pee and poo on the grass because he does not want to step on the wet grass. He steps on wet soil but he potties on the grass only, not soil.

I may have to train him to potty on the soil as well. I did this so I don't have to use pee pads. What can I do about this?