When school starts again, wait a few weeks to find out if your son will be placed in some classes that challenge him. Also, is there a gifted program at the school in which he could take part? You may discover that the curriculum in fifth grade turns out to be sufficiently challenging to keep him from being bored. If not, you need to request a conference with his teacher and a counselor to discuss what can be done to challenge your son.
The responsibility for challenging bright students does not just belong to schools. Parents must also provide stimulating experiences. This summer, why don't you try to help your son find a hobby or interest that really intrigues him. Then, he could devote considerable time to learning more about this topic, whether it is collecting stamps or studying rocks. When he returns to school, he could use and build upon this recently gained knowledge in his classes.
Don't jump to the conclusion that the teacher your son will have next year is not going to be a good match for his learning style. Just because a teacher is not a good choice for one child in a family does not necessarily mean that the teacher will be a poor choice for another.