Once your son has mastered carrying the pencil back and forth, add another small thing for which he can be responsible and earn a reward. Go slowly, adding one thing at a time and consistently giving your son the rewards he has earned. Be sure that you praise him when he is successful.
Make sure also that you make it possible for your son to be successful: Give him a bookbag in which to carry his things back and forth to school, a special holder for money and a certain pocket in the bookbag to carry it, and so forth. You need to teach him those basic organizational skills and give him the tools he needs to be successful.
You don't mention any other issues your son is having. Problems with organizational skills can be a characteristic of children with attention problems and you may want to talk with your son's teacher to see if there are other signs of this.