My son repeated K. He has a December birthday, and was diagnosed as being dyslexic and dysgraphic. It was the right decision for him, but it truly depends on each individual. Young age on its own isn’t necessarily a problem.
Your son only just started 8th grade. Honestly, I would think hard about having him repeat 9th grade because of his size or immaturity. He is going to grow and mature a lot over the next two school years. He is doing well academically, so it seems his ADHD is well controlled. If he continues at the local high school for 9th and makes a lot of friends over the next two years, be sure he is on board with the idea of switching to private school. It could be a case of “it ain’t broke so don’t fix it.”
@cam2932 my eldest started college at 17 and had no problems. Her friend group tended to be on the younger side in high school and all went to college aged 17, so I do think it’s a case by case situation.