I have seen this kind of behavior in my sons in various ways. I would recommend working with the teacher and the school counselor. I would ask if your child finds it hard to focus when there is so much to look around and see or hear (kids to talk to).
My sons, sometimes, needed to be at a desk away from the classroom windows or closer to the teacher's desk (where the teacher could get his attention redirected more quickly).
The most effective incentive I've found is making the rule that homework has to be done first (usually after a snack) before the TV is allowed to be turned on (or video game). Ask for your son to bring his homework to you when he says he is done. Give him a compliment FIRST. In this way you are being positive about the homework and not discouraging (with "what took you so long" or "I can't read this"). My favorite is a simple "Thank you so much for bringing your homework to me. It means a lot to me." After the compliment, you can proceed to "oh, I see you didn't work on this problem, what happened?". And don't correct all of his mistakes. Most of the time (like you said "he's very smart") it just takes some slowing down ("show me how you got this answer").
I'm guessing here, but he may be smart enough in math that he finds it boring and not challenging. That's a question for the teacher.
Way to go, mom of a second grader!
Good luck!