I don't know if this should be posted here or in the training forum, because as it is about a behavoir thing, I'm asking for training help. Sorry if I picked wrong.
Duke - American White Shepherd, intact male, 9 years old
Tucker - Labrador Retriever, neutered male, 4 years old
Other dogs in the house
Luc - Labrador Retriever, neutered male, 4 years old
Fathom - Border Collie, spayed female, 1 year 1/2 old
Riley - American Cocker Spaniel, intact male, 1 year 1/2 old
Duke and Tucker are owned by my mom, who I live with. My dogs aren't involved in the main part of this situation.
My mom took in Duke in June. We found him on the highway without a collar. We tried to find his owner, put out ads, and such. Two weeks passed and no one called for him, no one ever put out lost dog ads, etc. My mom decided to keep him. He greeted and got along with our dogs well until last week.
All of the bigger dogs were out in the yard, barking at a neighbor dog, running along the fence, sniffing the corners, etc. The neighbor dog was barking back. Duke jumped on Tucker. I didn't see it happen, but I heard it. Tucker didn't get hurt, but he had 5 wet spots from his neck to his back leg where Duke was mouthing him.
The day after that Duke kept tabs on Tucker. If Tucker moved, Duke was watching. If Tucker left the room, Duke followed, but stayed away. The day after that Duke cornered Tucker in the hall and jumped on him, again, no one saw how it started and there was no noise until Tucker started shrieking. Tucker wasn't hurt.
The past two days Duke has only zoned in on Tucker if the dogs start getting riled up, like play fighting, playing with toys, all wanting to go out, chasing each other, etc. When this happens my mom has cornered Duke off in one part of the living room and Duke calms down within 5 minutes. This has only happened twice.
Today everything was fine until I left. Duke, Tato, and Tucker were all in the hall by the door to the garage, and Duke jumped on Tucker. Tucker wasn't hurt.
Tucker is not fighting back at all. He is just doing a combination of crouching/cowering, trying to get away while screaming like a fire engine.
Also, last night everyone but Tucker went outside and on their way off the porch, Duke, Luc, and Fathom got a little pushy with each other. When they got off the porch Duke snapped at Luc. Luc snapped back and Duke walked away. This morning when they all walked out to do their business, usually Fathom will go pee and Duke will pee in the same spot right after her, today Duke went over to Fathom, Luc padded over with his tail up and when Fathom walked away, Luc marked the spot and Duke walked away. Duke went to pee on the fence and afterward Luc went to the same spot and marked over it.
I'm a member of another dog forum and the people that I've been discussing this with has mostly come to the conclusion that this is a dominance thing. This is splitting my family apart to two sides. Me, my dogs and Duke on one side and then my mom, my brother, and Tucker on the other. My mom wants to rehome Duke because she can't handle Duke attacking Tucker. I want to try to help the situation. I want to know if there's any thing I can do training wise to help. I thought of a removal technique orignally, but Duke doesn't zone in on Tucker that often at all. They can and do sit/stand/lay next to each other just fine. They can be in the same room fine. It's just when Duke gets riled up or something. The only signal is that Duke lowers his head so his neck is straight, he keeps his ears up, alert, and he pants. No lip curling, growling, barking, hackles raising.
So thank you for your time and any response you have. Is there something I can do?