You seem like a concerned mother who pays attention to what goes on with her child.
I read your post a couple of times and thought about what you had written. Given that he is ADHD and that you did an eye exam at home and he only missed one letter on the bottom row, it is entirely possible that the eye screening at school was not accurate. It may be that he has a visual processing deficit, which has nothing to do with visual acuity and everything to do with how the brain processes visual information. If the screening was done using a standard vision chart, I can understand how this could be a visual processing deficit.
With his diagnosis of ADHD, did he go through educational evaluation testing that might reveal any processing deficits?
Your son is nine, so he is old enough to be able to talk to you about his vision. Assuming that his vision has been screened in the past without problems, you could then ask him if he ever thinks that he has problems seeing, either things close up or far away. If he says yes, ask him to explain what he means so that you have more information.
You might want to ask him if numbers or letters seem jumbled or jump on a page. My son is ADHD and has processing deficits and a superior IQ. He's now almost 21, but when he was in grade school, the visual processing deficits did pose a challenge for him. He had to read larger sized print and use a ruler or guide to follow print. He had some timed math tests in second grade that had to be formatted on two pages instead of one so that his brain could focus on things better; it was his brain, not his vision, that was holding him back.
After you talk to your son, if he has support services at school due to his ADHD, I would recommend that you talk to that person and explain your situation. If your son is covered under your state's special education laws, the school might have to pick up the tab for another vision evaluation as part of his support.
I'd be very curious to know what happens.
Good luck and try not to worry. As mothers, that comes with the job, but worry doesn't do much to solve problems...kudos to you for posting here to find an answer.
Audrey
I called and its too late to get the vision insurance. I'm going to have to do this out out of pocket.
I guess I'd get the vision insurance tomorrow, and then go ahead next week and get your son an eye exam.
Put the question on the Med help eye care forum ...good luck