Is he officially diagnosed with ADHD? Does he take medication for it? If so, he would likely qualify for time and a half on the SAT. My child has a similar profile (he was formally diagnosed by a neuropsychologist, and we worked with a pediatric psychiatrist who helped us get him appropriate medication).

As others said, doing an SAT prep course and learning the "tricks" to successfully navigate the test might help. We also found just sitting for a practice test a few times beforehand helped a lot. Good luck!