hi IQ immature strongwilled child - grade retnention?

My husband and I have been married for fifteen years. We have only one child. I am a stay at home mom with a home based business. We strive to have a happy healthy home and we spend as much time with each other as possible. My son is 6 1/2 and in first grade at a private school. He is disruptive in his class, talking out of turn, not staying in his seat, not paying attention. He is aggressive in handling conflict, yelling at his classmates and pushing to make a point, thus alienating them. He is very bright, testing in 97th percentile on the SAT in Kindergarten, and works harder and has better comprehension of subjects than his classmates. He dallies doing his seatwork or does it too quickly often not following directions and thus is sloppy unless constantly reminded to focus on it. He is stubborn, defiant, outgoing, physically coordinated, always wants to be in charge. He is less mature than his classmates. His birthday is in October and we did not delay his start in school. He entered this school in K4; this is his third year there. He has had the same problems every year but it’s getting worse. At home we have the same problems with disobediance, disrespect, some lying, defiance, angry outbursts, and pouting. His father and I are firm and loving with him just as his teacher is at school, but our efforts seem to no avail. He is active in Cub Scouts and sports year-round. He is disruptive or focuses poorly is all his activities. He wants to talk or gets distracted through the whole activity. He is becoming labeled as a problem child by all who know him. His teacher has suggested retaining him in first grade thinking his problems stem from his immaturity. I do not want to retain him because he is academically very bright excelling in math and reading. I have thought moving him to a different school next year with smaller class-sizes and a first/second grade combination where he can be with younger children but still do second grade work might be more appropriate for him, but if we can’t get his impulsive hyperactivity aggressive behavior under control the same problems will occur. We are pursuing ADHD testing as well as physical, vision and hearing tests. During the summer he will participate in a home-based summer-school/camp project with a half dozen friends of mixed ages, and in swim team and swim lessons. I love my son dearly. He has a lot of potential. My question is: do we retain or promote him at the same school or a different school? Is ADHD a likely cause of his problems or something else?

A Mother in the CA High Desert