The research on “green time” for managing ADHD symptoms is suggestive rather than compelling. There is anecdotal evidence, and there are a few small studies. However, I do believe that spending time in nature is a positive thing for any child, and especially for children with ADHD.
A patient of mine, a 10-year-old with ADHD, could barely sit still for five minutes. When his grandfather took him fishing, he sat quietly in the boat for hours. Many parents have told me that the best time they have with their children is when they are out in nature.
There is no way to predict whether green time will help your son manage his symptoms, but it is definitely worth trying.