Over the years we have worked with two different psychologists, a psychiatrist, and different medical doctors. He's been diagnosed as ADHD and depressed. We have tried medications with no success. Any suggestions of what to do or where to find help would be greatly appreciated.
You point out that he has been diagnosed as depressed and that he has ADHD, but that the medications have not worked. I would suggest that you take him to see a psychopharmacologist (a physician who knows about medications that affect mood and attention). Make sure it's someone who has had lots of experience treating adolescents. This way you can get a second opinion about the diagnosis and the appropriate medication (or combination of medications). If a doctor is able to find a medication that will help "lift" the depression, then Sean may be more willing (and more able) to take the tests that will help answer the LD question. He may have ADHD, but he could also be having difficulty focusing because he is depressed or distracted by worrisome thoughts. The consultation from a psychopharmacologist will help determine whether his difficulty with attention is a cause or a symptom of his problems.
It sounds as if there is a cycle of problems that needs to be broken in order for your son to improve in school. If the school has not done a thorough evaluation to determine the need for special education services, that should be done now. Unless something different happens in school, he's likely to continue to stay in neutral or regress. If he's interested in something right now, (like cars or electronics, etc.) try to find a school or program that will take advantage of this interest. Sean may be more motivated to learn if he's in a vocational program or a program that offers internships in the workplace. Moving up to a higher level or repeating the year aren't good solutions unless there's something different or special about his education.