House Trained Dog Pooping Inside After Moving?

My gf moved in and brought her intact male Miniature Schnauzer, Louis, into my house where I live with my Dutch Shepherd Dex. It's been nearly 3 months. For the first month the Schnauzer had no issues going outside in the back yard. Lately he has been peeing and pooing inside in the living room. We are shocked because this dog never did it inside, since he was 3 months old.

He is about 1 yr and 3 months now.
The big dog bullied him at first, but now they just play. Sometimes the big one will assert dominance but nothing too unusual in dog behavior.
We don't know what's happening and what to do. Is it his environment change, the new dog, is he scared to poo outside? Any help would be greatly appreciated.

By DexandLouis