Here are some important things to consider: Your son should have an individual educational plan (IEP) which spells out his learning abilities and needs. You say that the school modifies his grades so that he passes, but he's not learning much. Are they modifying his curriculum (the materials and techniques they use) so that he can learn to the best of his abilities? If not, something needs to change, especially if he's been put, as you say, in a "no hope" category.
If you don't feel that his IEP accurately describes your son or his needs, ask that he be re-evaluated or that the plan be re-written so that it contains goals and objectives that are realistic, that can be measured, and that address his needs. Although it may be hard to accept, your son has some limitations. No one can tell exactly what his limits are, but he will have difficulty learning. You have to work with the folks at the school to decide what's realistic for him. He may need a curriculum that focuses on self-help and pre-vocational skills (even at a young age), as much or more than he needs an academically oriented program.
If he has a a diagnosis of ADD, is he taking medication for this condition? If he is, is the medication effective? Is the doctor who prescribed the medication experienced with lots of kids with ADD, and is he or she maintaining contact with your son's teachers to monitor the effectiveness of the medication? If medication is not being used for some reason (75-85% of kids with ADHD benefit from a proper dose), then is the school doing things that address your son's attentional problems (providing structure and organization skills as well as strategies that teach your child to focus better, etc.)? If not, you will want to have these modifications built into your child's IEP.