Some strategies you may want to consider would be having your son do one thing at a time and take frequent breaks, if needed. Have him then check and recheck his work after he is finished. Once this is done you can have him apply what he has just learned, to a real life situation, so the subject can become more realistic. Another option is to sit down with your son, if you notice he is rushing through his studies, and say "Stop and take a deep breath. Think about what you are doing right now and not what you plan to do next." Finally, consider giving him a reward for completing his studies without rushing.'
Knowing how to help our children be and do the best they can, can be a challenge. The fact that your son is confident is a very positive thing. However, it is important that you teach him the importance of slowing down and doing his very best on tasks. Let him know that if he rushes through things he is going to be more likely to miss important details.