Hello....What grade is your son in? Is the main problem that your son forgets to bring home his homework or does he forget to complete his homework once at home?
Ok..This is going to require the cooperation of you as the parent and his teachers to keep your son on track. As for your job, you need to remind him to do his homework and check that it completed daily. Initial his planner that you have seen his homework and it was completed so that the teacher is aware that you are helping out in the problem. Have your son place his completed work in his folder or backpack nightly.
When he is at school, you can request that his teachers initial his assignments that he has written down daily in his planner. They need to check to make sure he has his work before leaving class for home. This takes time but your son needs it. To establish this routine, meet with your son's teachers to develop this plan of action.
It seems like your son is not bringing home his work or turning it in because he is simply not interested in school. He needs to be more motivated. The teachers can create a behavior contract for your son. Mention this to the teachers and they can help create one specific to your son's situation.