How to train an adult male Lhasa Apso?

I recently adopted an adult male Lhasa Apso who is 7 years old from the Humane Society. He is very bright (knows how to shake hands, sit etc...) and he is a good dog but he is aggressive and does not like to listen. He also marks his territory in our house as we have two other dogs (one female Lhasa and one male Shih-Tzu). He is not neutered and cannot be neutered as he had kidney problems as a puppy and had to have a liver shunt. How can I train him to not mark in our house and to cut down on his aggressive behavior? He is an excellent watchdog and he loves to play and receive affection.