Well, three things to consider on that one:
(1) Unless you have a reason to believe that there has been an insult to the brain (injury, accident, disease, etc.), there is not much that neuropsych can do from a starting point of ADHD and/or ODD. A neuropsych is more geared toward determining which area of the brain has been affected, to what extent, and then working out a treatment plan to help the patient overcome his/her insult. (As an aside, another pathway toward brain damage is chronic drug abuse)
(2) If you DO decide to go with it, you need also ask the question: How will this help my grandson EDUCATIONALLY? You are pursuing this evaluation from the school system, so it has to relate to education/learning in order to be relevant. The testing would be (as you know, I am sure) very expensive, so having a clearly defined question will be vital. In my opinion, this will be difficult to do.
(3) . If you are thinking about this, your first step may be to set up a sit-down with a neurologist rather than a neuropsychologist. Thje neurologist can at least give you some options or help you to pare down your interest into feasible chunks for the tester to examine - or even tell you if you are on the right track. Some of the answers you seek may not be possible, and a neurologist would be the gate-keeper of all such information.
I hope this helped. I am a little tired, so fear my response may have been frazzled. But to recap: attorney for the school, neurologist before the testing. In general however, and knowing the little bit that I do about your grandson, I would lean toward not
feeling that the neuropsych would uncover anything useful to your current situation.