Your son may have learning disabilities and need special attention in order to do well. He may be very bright, but learn differently. You need to make an appointment with the school psychologist or an educational psychologist outside the school to have him evaluated. The sooner you do this, the better. Younger children respond more quickly to educational changes than older children do.
The first thing you must determine is why your child is having difficulty learning. You should check with his school principal to see if academic testing can be done through the school to isolate the problem. Once the problem is identified, seek help from the appropriate professionals at his school. If the situation is beyond the school's capability, you have every right to approach your local school board for additional help. The "No Child Left Behind" law makes it clear that all children needing help in learning, will receive it. Be as pro-active as you can. Addressing this problem now, while he is young, can make a world of difference in his academic development.