Hello..here are some suggestions to help your son. You already may be doing some of them.
1. Set proper expectations for your son. Make sure they are high (but not too high) and he is aware of them. Communicate with him often about meeting the expectations.
2. Along with expectations, have your son write down his own goals in all areas and how he plans on achieving the goals. For example: I am going to get better grades by studying more and doing my homework well.
3. Show your child that you think school is important. Ask him how his days are. Conference with the teachers often. Make sure that he has an appropriate environment to complete his homework. Keep up with his assignments. Stay positive about school even when he is not doing well.
4. Support your son's learning style. He may be a more visual, auditory, or kinesthetic learner.
5. Provide much encouragement and praise.
6. Encourage resiliency so that your son will be ready for whatever life throws his way. Teach him to solve his own problems with your support. Focus on his strengths. Make sure that he understands that mistakes will happen and he can learn from them.
Here are some additional resources that may help:
Bright Minds, Poor Grades: Understanding and Motivating Your Underachieving Child
by: Michael D. Whitley
Motivated Minds: Raising Children to Love Learning
by: Deborah Stipek and Kathy Seal
If you still feel like you need more for your son, he may need additional counseling from a professional who can provide him with intervention techniques and strategies in dealing with his lack of motivation and possible low self esteem.
Hope this helps you!!!! Have a GREAT evening.