Dog pee is poisonous for piggies, so the best thing is to fence off an area for the piggies. Make sure that any run has a firm top - cats and birds of prey can also pounce within seconds.
Please be aware that most piggies are very frightened in new territory, especially wide open spaces, so have a beach towel ready to put over the top and hides not spaced far apart. it will probably take a few trips until they come to grips with the big outdoors.
Ideally, you can find a guinea pig rescue within your reach that allows you to bring Isaiah for a round of "boar dating", so he can choose his new friend. That increases your chances of a stable bond enormously.
