My 5yr old son is being mean to small animals...how do i get him to stop?

My 5yr old son has lived with him mother all his life up until about a month ago, when i filed for emergency custody and was awarded custody of him. Today when i picked him up from the sitters they informed us that he has been being mean and cruel to their cats and dogs. They said they talked to him about it and we tried to talk to him about it and when we asked him if he thought that when he slammed their cat up against the wall it didn't hurt the cat he said no...and when he was throwing a ball at their elderly dog he thought it was funny. I was shocked by his answers and punished him for what he had done. i am at a lose as to what to do. Now mind you while living with his mother he was allowed to watch movies such as Child's Play and knows all to well who the Chucky character is and believes that he is real. She is also seeing a convicted man, since Oct of last year, who was sent to prison back in 1991 for murder 2 on an 18 month old child and seems to have no issues with what this man has done. My son has told my wife and i that this man has been abusive to him but will not tell his mother anything because when he tried a few times before she told him she wasn't going to make him leave because she loves him. I am looking into getting him counseling ASAP but i guess i am also looking for a few answers or helpful tips to help deal with this in the meantime....