11yr old son behaviour normal? He is extremely forgetful, lies, can't be trusted to do simple things etc?

Maybe he needs some attention from you. He might be going through that confused stage of puberty when you feel like "Nobody understands me". Everyone goes through it, his may need some extra work. Just try talking to him. Is his father around? Maybe he can talk to him. Do you spend more time with your daughter then you spend with him? Does he have friends at school that treat him right? These are the questions that you need to find out. This could be normal tween behavior, OR it could be something else.