Limit TV until schoolwork improves. While you can't make your child study, you can increase the potential study time. Eliminate all TV and video game time on school nights. Explain that you are doing this to help him better structure his time. Consider adding incentives for improved school work.
Most children respond better to incentives than disincentives.
Ask your youngster what he thinks would help. Some good incentives are taking your child to a favorite restaurant, amusement park, video-arcade, sports event, or the movies. The payments can be made weekly based on the teacher's progress reports. A's, B's and C's can receive a different cash value. Rewarding hard work is how the adult marketplace works.