Pet Sitter

We had our 2 cats for four years and they are a mother daughter duo. They love each other and get along. Both are calm. We got a male ginger cat that the other two cats are not getting along with. Its improved but theyre still hissing. He also has potty issues. He is sweet but he bites in a loving way, including humans. Our cats are taking it as aggressive. Even for us, its too much. My husband wants to return him but I feel so depressed thinking about this because I feel committed and loyal to him for life. I asked him to give him a chance and see if a trainer can help acclimate him. I am so attached to him and I dont want to lose him.