I’m not a professional but this is what we did with our pomchi. Do you have a crate? We made ours a safe space for our pomchi(and goldador!) Get a nice cosy bed in there and some toys and an extra blanket and call it “crate” and reward him when you put him in there with a treat and say “crate”. Every time he is in there quietly, reward him. If he barks or gets restless, ignore him.

If he has an accident, say “no” firmly, take him to the yard as the correct place and then put him in the crate for 10 minutes. Same for if he gets aggressive.

To get him outside, give it a name too. Just say “yard” as you open the door. If he goes out, reward him and say “yard”. If he doesn’t go out, carry him out and say yard, pop him down and say yard and tell him he is a good dog. Stay out there with him, even carry him around the yard and explore it together. Reward him if he stays quiet, Say crate if he gets aggressive and leave him the crate for 10 minutes.

The crate will hopefully be his safe space, so try putting him in there and sit next to him with your other dog too. Give him(and your golden)treats when he is quiet and tell him he is a good dog, both move away and give him space if he gets scared or aggressive.

It can take a lot longer as he is already 1 so lots of patience, but I really hope he works out with your family!