While there may be something to be said for challenging new material to capture his attention, if he has ADHD he may need more than challenging new material... he may need or benefit from behavioral therapy and/or an IEP/504 with specific supports/accommodations/interventions.

You may have read this elsewhere on the forums... when considering acceleration, the Iowa Acceleration Scale (IAS) is a helpful tool.

The Fin, Fur, and Feather Bureau of Investigation, funded by the U.S. Department of Education may be of interest to work on at home.