HI and welcome. What is his diagnosis? Has his vision been checked?
Kids that age don't have a really long attention span anyway, although meds can help focus, they won't tremendously increase the attention span. Does he have any Ld's? It could be any number of things, besides meds.
A child may HAVE ADHD, but it is not what they ARE. Never tell a child they ARE ADHD.
When you say that he cannot focus to read the correct word, does that mean he is reading them out of order or the wrong place on the page?
My ADHD gifted son had problems like that, but it was not related to medication. He has problems with getting his eyes to focus on the words correctly. He was seeing the words in double vision and sometimes they appeared to move around on the page. He did not have issues with phonics and early reading, because I think, the font size was large. As the type got smaller then I noticed that he would read the last word first or skip words or start in the middle of the line. I had to put an index card under the words for him to slide along the page.
We got our family doctor to look at him - he thought there was a slight chance of lazy eye. We got a referral to an optometrist who does vision therapy. This has really change things for my son. He did 2nd grade last year - testing at the beginning of the year he was 6-9 months below grade level, at the end, (most of the way through the vision therapy) he was 6-9 months ABOVE grade level. (As high as the testing goes.) He also has a longer attention span, can read on his own now and can even throw a ball better. Before, his reading patterns were bordering on dyslexic types of reading. (Letter reversals, out of order reading, better at sight words than phonics as the fonts got smaller.)
This has made a huge difference for us - it may not be the thing that your child needs, but it couldn't hurt to check and rule it out. Be sure to go to a doctor that is accepting of vision therapy - some think it is a bunch of hooey, but I have seen the difference that it made in my son. My son does have 20/20 vision, his eyes just were not able to work together.