I feel your pain and am sorry that happened. My spouse and I have given up on trying to assist our in JW family out of the cult. Some do very minimum activity in WT, so we just keep our conversations on family issues and if anything is brought up, we change the subject.
It sounds like your family got a hold of him and trash talked you. Even if your family knows nothing, they will make up something. It's the JW way. I have a close friend that is studying with the JW's. I am just kind and keep off the subject of the witnesses. I figure, if he stays in, it will just be a matter of time before he is disappointed and offended by something. Let em enjoy his cult for now. I have a feeling he'll be back one day.