You say you have talked with his teachers and counselors. Did those meetings result in a plan? Was your son involved in that plan? If the answers are no, I would suggest going back to the school and doing just that.
Homework: Is there a set time and place free of TV, computer, stereo, etc.? Does he have to use that time reading if he says he doesn't have homework? Do you or your partner spend some study time with him talking about his work and checking it over? Where does the homework go when finished -- in his folder, in his backpack, near the door? Many bright seventh-graders have no idea how to organize or how to study. Does he need some help in this area?
You speak of punishment and rewards, but not of consequences. Are they logical, immediate? What happens if he fails seventh grade? Have you decided as a family and made clear to him that he will be spending summer going to summer school -- not on vacation or being with his friends? That might help!