I think its wise of your son's pedi not to diagnose something he is not an "expert" in.
Not sure about where you live, but is there a teaching Hospital around? Often times Hospitals have programs that treat / diagnose children.
I would also do a lot of research in finding a Child Psychiatrist - make sure they are board certified. You can always ask to be put on a "cancellation" list at the various Psychiatrist offices. I too travel far to see my daughter's Dr. - often times that is the case.
So, I have a few ideas ......... As far the move goes, I would try to not talk about it very much in front of him. When he does here you talk about it, just make it matter of fact, and exciting. Like "It will be so great to be with Daddy again in our new town. We can go on adventures and learn about our new place" - things like that. I would also buy a few books that can help ease the transition and help with worry: Here are a few ideas:
Homeschool.......well, I tried it with very little success with my very ODD daughter and found it created more battles than it solved. Also, my daughter struggles socially and I found removing her from school alienated her even more, and frankly I couldn't hack the school work battle with her.
It may work for you guys, but it may not (you don't want yourself going completely bonkers!!).....
Your son should qualify for an IEP and specialized services through the school district because of his diagnosis. Now, depending on where your moving, services can vary, but he would be entitled to some. IE: Extra time taking tests, 1 on 1 additional help, preferential seating etc......
Here is a helpful link to look at regarding school: