Found out 7 year old attacking puppy

We got a puppy a few months ago. He's just about 6 months old, he's a retriever. Very well behaved now and well trained. I noticed that the puppy was quite aggressive to 7 year old, josh.Josh would wake up in the morning and then so would my partner. It would always frustrate me to hear the dog getting told off and put in his cage. When Josh goes to bed he's absolutely fine, there's never anything wrong. For a while I thought maybe Josh wasn't playing right with the dog, so me and partner would monitor and instruct Josh how to do things properly.It continued. Me and partner would fall out over it. She would see the dog being violent with Josh and I'd be just wondering why it's just him.This evening we decided to record them together. I hooked my phone up to the TV in the bedroom and hid the phone in the living room and left them both.This is where things go downhill. What we both witnessed was shocking and disgusting. Josh would antagonise, hit and beat the dog. Grabbing him by his tail, pulling his back legs. He grabbed a remote and hit him on the head a few times. Put him in headlocks. He'd run a long the sofa if the dog started to react. It was dreadful.We pulled Josh into the bedroom and made him watch the video. They went on for about 40 minutes. This is just one night. He knew what he was doing was wrong because before we showed him the video my partner asked him if he was playing nice and if pulling a dogs tail was acceptable which he made clear he knew it wasn't. When watching the video we asked him why he was doing this and his only response was "I don't know". He would never pull this shit in front of us.Well we've completely stripped his room of every last toy. It's going completely neutral, will be painted white tomorrow. But he doesn't seem bothered by any of it. All he's got now is books, he's not going to be able to watch TV. All we'll allow is reading or drawing.What should we do about this behaviour? Its not just this he'll answer back at everything.We think a lot of his behaviour can stem from his grandparents due to him always coming back an argumentative little shit and then his behaviour will improve throughout the week but they'll always deny any wrong doing and we rely on them to look after him. His grandparents are his dad's parents.What do we do? via /r/Parenting http://ift.tt/2eptmcy