Welcome to YT! This is a great place to find the answers to your questions and hopefully someone more knowledgeable than I am, posts.
But I will try to help. First off, I think you are doing the right thing having him neutered and maybe this will help if he is trying to establish dominance. I had a trainer come to my house last year and in the course of conversation he was telling me about the Rottweilers he has dealt with. He primarily trains them as guard dogs and he said that he trains them to be protective of women and children. When he got his huge Rottweiler out of the truck my heart stopped for a minute but he was the most gentle dog and don't you know, came right over to me after sniffing my collie and sat right next to me touching my leg. I felt so safe with him around and wanted to keep him! Anyways, the point of my story is this.... I asked the trainer how he taught them to be gentle around women & children and he said one thing was that he walked them behind a baby stroller and never let them be in front of you but especially behind the stroller or child. Maybe you could walk him with your niece on her bike or something? That's just one suggestion, I'm sure others will have more.... Good Luck!