Becareful, as Helen says, that there are no toxic plants.
Wherry is more of a trampeler than a chewer so i'm not too sure but could you try a little fence around the boarders (if you have flower beds) and again, making sure there is plenty else to do in the garden?
I did just see something in the garden center, pepper something, to keep dogs and cats away but I don't know what it is or if it's mean/effective etc.
To rid Hostas of slugs they suggest you boil a big bucket of water with loads of garlic in it, leave it to infuse/co cold and chuck it over the plants. This may help deter him and obviously it's not bad for the plants.
I think that unfortunatly having a nice garden and a cocker is somewhat of a difficulty