First, establish clear house rules regarding studying. When he takes a break after school, he must study before he engages in any fun activity. He is not losing privileges; he is delaying getting privileges until he does his homework. Set aside time each day to study. Make sure he is studying in a place where there are no distractions. Instruct him to turn off his phone, television, music, etc., and minimize other distractions. If he struggles initially, you can break down the tasks. He could study for short periods of time, and then you can gradually increase his study time as he achieves success.
Have him study initially for 30 minutes and then take a 10-minute break. If 30 minutes is too much, try 20. Gradually increase his study time — but continue to alter studying with short breaks. When he appears to be having problems concentrating, remind him that he only has a certain amount of time left before he has a break.
Also, make sure that your son writes down every assignment on a to-do list so he won't forget. Many schools post daily assignments online. If they are available online, and you have a computer and a printer, then have him print off his assignments. As he completes each one, have him cross off each "to-do." Not only will he know what assignments he still has have left, but this also gives him the satisfaction of crossing each one off the list. Make sure you give him praise as he accomplishes his tasks. This is vitally important. Again, he can take a short break to reward himself after completing each assignment or task.
In the classroom, you could request that he sit in front of the room (if possible) so his attention is on the teacher. He should also be encouraged to take notes to help him remember what his teacher says. He should review his notes at the end of each school day. This can be done as part of the homework process. It doesn't have to take long; just 15 minutes of reviewing notes can be effective in reinforcing the day’s learning.
If the issue is more than just compliance, talk to your son's teachers and let them know he is having difficulty with classes. Ask them for suggestions regarding what he can do to improve his grades. It would also be good if he could talk to them himself and identify what problems he is having specifically.
Your son could also ask to complete extra-credit assignments and inquire whether student tutors are available to help him with difficult subjects. His teachers may be willing to tutor him as well. In addition, if he continues to have difficulty, you should let his school counselor know that he is having problems with his schoolwork.
As for you, maintain frequent contact with your son's school. You will likely have to keep close tabs on his progress until he is able to work independently.