You have articulated the frustration many of us experienced at the beginning of the gifted journey...why isn't there a clear path, schooling options, obvious way forward. There often isn't. There are very few school placements that are for gifted, rather than high achieving students, and even fewer for twice exceptional students who need additional supports. The more individual a students needs then the more difficult it is to find a simple path or place for them.
I am not in the US but the provision of gifted services is variable worldwide. Some are fortunate and find a flexible school willing to try different things to make learning better, it will never be ideal. If your nephew's school is listening and trying they may be able to do more with guidance. Unfortunately it is down to those who love and care for each gifted child to do the reading and the research and work out what each child needs. That can be done in conjunction with a slew off professionals who also provide guidance, psychs, neuropsychologists, OTs, etc and your nephew is on the way.
Reading about Twice Exceptional children is a good start, also being informed about acceleration in all its forms as this encompasses many interventions for gifted kids. There are others much more informed about ADHD who may chime in but for acceleration start with A Nation Deceived which you can download here.
http://www.accelerationinstitute.org/nation_deceived/get_report.aspx