House Trained Dog Pooping in House?

My mother owns a 1 year old Boston. He is an intelligent dog and has been house trained for a long time. He typically rings the bell or sits at the door. His day to day activities include waking up early around 5:30AM to go outside. He is walked a long distance and does his business. My mother gets ready for work and lets the dog roam around the bottom of the house. She gets home around 2PM and takes him back outside and before bed time which is around 9PM.

I just came home from working out of state and love hanging with him, we have a great connection. However, we are starting to wonder if this could be a dominance thing or him acting out since another male is now living in the house. It is just my mom and him that live together. Recently, 2 weeks ago he got snipped. Also, here in Pennsylvania it's quite cold outside, but that has never stopped him from wanting to go outside and play in the snow. Why is he pooping and peeing in the house? What is causing this behavior?

Thank you.

By Dusto