Wow, I hardly ever do this, but gonna disagree with Rockrose on the Adderall issue. As you said, "But this was happening long before the adderall." The rule of thumb is that you deal with the mood disorders first, and then the ADHD. You don't ignore one or the other. (New ADHD Medication Rules, p 97)
Wow, I feel for the little guy (and you). When he is talking to these characters - is it all day long or after the adderall has worn off. Regular adderall is only good for 4 to 8 hours. Start charting these behaviors to see when they are happening.
And yes, you really do need more then just a pediatrician to deal with this. This is way out of their league. Do the docs who say he needs the MRI yearly have any references that you can use. It sounds like they have seen this before.
And you know in some ways he does sound like a typical 5 going on 6 kind of kid. They really do live in a slightly different world. They have enough intelligence to know what is going on, but no clue why.
I am almost tempted to suggest you start reading Ronald Dahl stories to him to show him that imagination is ok.
And maybe rethink the tv limits a bit if you can find the right show that actually gives a bit of science, etc.
Oh, I am also the CL here -
http://www.medhelp.org/forums/ADD---ADHD/show/175
Don't be afraid to post there for more info on ways to help him with his ADHD.