It's hard to add a lot to aeh's excellent advice, but I would just emphasize two questions. How situational is this problem? And, does your son himself feel like he's experiencing a problem?
It really comes down to whether he's having trouble responding appropriately to situations, or whether he's responding perfectly appropriately to inappropriate situations. Is he missing any instructions he actually needs, or just impatient with unnecessary repetition? Does he experience these problems when appropriately challenged, or only when dealing with teaching that doesn't meet his needs? If he is allowed to move during instructions, does that solve the problem? (Sometimes that's all a kid needs, but unfortunately, some teachers see that movement - which actually helps the child think and focus - as tuning out and disrespectful).
My BTDT advice is it's time to go for an ADHD assessment when the child himself is frustrated or upset with their own behaviour. When they want to make choices or do things, and they feel like they can't control their own mind and body to make those choices.
If the child is happy and functioning well in other environments that meet their needs, and only having trouble with this one particular classroom, then I'd look to fixing the environment, not the child. On the other hand, if they are having trouble accessing environments and activities they want to access, then consider looking to what also can be done to help the child respond differently.
