They think it's ADHD? What the heck are they waiting for? His reading is below grade level. They need to find out why and do something about it -- now! They are lucky he's a good, helpful, and affectionate child. If they don't figure out how to teach him in the proper way soon, they'll lose these wonderful traits.
If he's the youngest kid in the class, holding him back a year might give him a headstart, but they've already played this card. I'd want to know the following:
Does his immaturity get in the way of his learning? Does he have opportunities to interact with other kids his age outside of school? Does he have increasing opportunities to take on more responsibility? Are the adults in his life teaching him the proper way to handle social situations? Is he in, or does he need to be in, a social skills training group?
The school psychologist or guidance counselor might be able to answer these questions and provide the needed services.
The school should carry out a comprehensive evaluation -- including neuropsychological and reading. Be sure to request this in writing to the Director of Special Education of your school system. Expect some concrete answers soon. If you don't get them, seek outside help.