All children, especially seventh-graders, have to be taught study skills and organization skills. To teach a child, parents and teachers need to set up expectations and consequences -- both short-term and long-term. Let's take bringing homework and books home as an example. What happens if your son forgets to bring his books home? He should walk (bike) back to school and get them if possible. If he doesn't, he shouldn't be allowed to watch TV, play video games, etc. He should have to spend one to two hours reading or studying, whatever you choose. He may not actually do it, but just sitting in his study space for this time will get awfully boring for him. That is a short-term consequence. A long-term consequence might be that if he hands in all or most of his work for a term, then he gets to spend a day doing an activity of his choice. Natural and logical consequences of failure would be spending the summer in remedial or make-up classes.
Consult with the school counselor about parenting classes. Some classes are especially for parents of children with ADHD. If no classes are available, go to the library. There are many books with advice on helping parents with study skills and working with children on medication.
Your son is not sick. He is capable and I encourage to treat him as responsible by keeping your expectations of him high, urging him on, and learning all you can about how to teach him these important life skills.