What is the best way to deter future dog accidents in the house? (new dog)?

You have to be very firm and make sure he knows that it isn't acceptable. Then take him out side to show him where he's supposed to use it. Doesn't take them long to figure it out. But as long as you never let him know that you will not put up with it, he will continue to do it because he knows he can get away with it without no consequences.