You and your mother have done a good thing by saving that dog. He might have been abandoned because he was in such poor health, and that could be why he urinates inside the house. If you encourage him to go outside, and show him that you are not pleased when he does it indoor, he might stop doing it.
Your dog has not attacked the new one so far, but he might not like all the attention you give to the new one. It would be best if you make sure there is always someone to watch them. I think in time your dog will get used to having another dog in the house. Let them spend time together when you or your mother are there, and they could become good companions.
