What bred dog might be the dad to my pup?

My Border Pit Collie had one puppy past November. I thought I knew who the dad was because this (large) Pomeranian keep climbing the fence and coming in the dog door. My Border Pit Collie can't fit out that dog door and the other one was closed so she would stay in. I have relatives that stop by when I'm at work to get eggs or what ever. I'm thinking they may have let her out because this puppy is getting huge.

Just want some guesses as to what mix some other people think this puppy is.

4.5 mos old approx 30 lbs &12 ins @ the shoulder.