My 6 yr old is really mean?

I am a very loving and giving person, my ex husband is a very controlling, manipulative and abusive person. He doesn't seem to register other peoples emotions at all. Now my son is six and I have noticed some disturbing behavior. He bullies his little sisters sometimes into corners where they can't get away. He says awful things, he has hit a teacher, he screams like a crazy person when you corner him to ask him to stop misbehaving. I don't him at all- mostly because I share custody with his grandma (His dad is way out of the picture) and she doesn't him often. A couple weeks ago he walked out of a day camp program - across a busy highway and six blocks home, his grandma is just as perplexed at his outragous behavior as I am. When he says hurtful things I calmly explain that it's mean and ask him how he would feel and he goes through the motions of understanding but then reverts back to being really cruel to others. I ended up in tears the other day after he said he wasn't my son and I needed to clean my house (it's clean)- I explained to him that it was not nice at all to tell an adult to clean their house and he said "I don't care you, should do it then he gave me this weird smile and said "But I love you." He is acting creepy and out of control- I feel so out of touch with him - I swear there is NONE of my gene pool in my son- he is manipulative, mean and doesn't seem to register that others have feelings. The doctor said that he is ADD but I don't see what that has to do with being a very mean little boy. Seriously does anyone else have a child who acts this way? I am really kind of afraid of who he will turn into when he gets older. I don't seem to have any effect on him at all.