Your son seems to need more motivation to complete tasks. Since you desire to focus more on positive aspects being a gentle family, then you should reward your son when he exhibits desired behaviors without your telling him repeatedly and nagging him.
Focusing on what your son is not doing right obviously has no effect on him. So you must flip the script and really praise him for doing what is right. Maybe increase his allowance. Possibly buy him a new computer game or DVD after he has a good week of completing tasks. You must be consistent though. This process will not happen over night. Your son may be stubborn at first to change his ways. If you have another child, you could implement the same plan with that child.
Your main goal is to reduce the undesired behaviors and replace them with desired behaviors. Constantly remind your son when he has wronged and allow him to say what should have been done.
I hope that this has helped. If so, please ACCEPT so that I may be paid for my efforts. If you need further assistance, please let me know. Have a great evening!!!