Has he ever tried to eat one? My dog would sniff them and move on, never seemed to think of eating one. Other dogs' shits on the other hand...
We had a big stainless dog bowl under the outside tap which was always topped up. I used to wash it out frequently as slugs and snails gotta drink too.
Where do slugs go during the day?
Take my dog out into the garden for a pre-bed piss, and there are always loads of fat slugs on the grass, that I need to make sure he doesn't eat.