Here are some questions you might ask your son's teacher:
1. What are my son's strengths? What does he do well at in school?
2. What are his weaknesses? What subjects does he consistently struggle with?
3. Under what conditions does he work best: Alone, with a single "study buddy," or in a cooperative learning group?
4. Is my son completing work within the time frame you have set for him? Is he rushing to finish so his writing is sloppy or his attention to detail erratic?
5. Does my son seem to understand what he is supposed to do to complete a specific task? Is he paying attention to directions?
6. Does my son ever ask you or another student for help if he's stuck or doesn't understand what to do?
Once you get the answers to these questions (and others you think are appropriate), it will be easier to talk together to devise a plan to help your son. Some of the plan may involve accommodations (e.g., slower, simpler presentation of directions; shorter assignments in class; opportunities to take a brief break between periods of intense work). Other parts of the plan may require more direct interventions (e.g., If his work in class is sloppy and/or incomplete, he may need instruction/practice on improving his handwriting or in organizational skills).
If you son doesn't already have one, he should probably have a Section 504 plan outlining his needs and the accommodations put in place to help him. Ask your school principal or guidance counselor who is in charge of this. Once the plan has been created, it needs to be updated every year and modified as necessary. Have a look at Harvey C. Parker's book, Problem Solver Guide for Students with ADHD for some concrete suggestions about the kinds of things you can ask for on a Section 504 plan.