As a mother that raised a houseful of sons, my opinion is boys should not be in a formal classroom until about the age of ten. Most just can't sit still that long. You have said he is smart little guy. You don't need to worry about him falling behind. I would just talk with him about rules and how you realize they are hard. Let him know you understand how hard it must be for him to be still and listen. Make it a point to show understanding but also be firm. The rules at school must be followed. If the school is punishing him don't double punish!!! However, if the school is taking a passive attitude with just charting the behavior and letting you know, the ball is in your park.
If that happens, clearly a contract is in order. One that you and he signs stating what will happen if he breaks the rules at school (like no tv or no video games). At his age, I would make the no tv or video game for just one night. Taking it into a full week leaves nothing for the next day...lol...I like the contracts because then you need not actually punish him. You just point out the contract and he will know. Please don't get into buying off good behavior...That is a big no no. He doesn't get rewarded for what he should be doing.
Just my two cents.