MWM, I understand what you're saying. And Janna, I didn't get any vibes about, "Oh, what a relief it's autism and not BiPolar (BP)" - what I DID get from you, is "What a relief we've got some clearer answers and maybe have a better chance now of helping him."
Also, in terms of how the child perceives himself - both my boys embrace their autism as something about themselves that they value. I think if they had BiPolar (BP) it would be more difficult for them to do that. Also, correct me if I'm wrong - in BiPolar (BP), it's a lot harder to maintain stability than in autism. My understanding - with autism, if you can control the environment, you can control a lot of the problems; but in BiPolar (BP), it's a matter of understanding the changes within, and these are far less predictable.
Not that I am saying one is better than the other; they ARE different and have different problems. And whichever it is, being more certain of the diagnosis makes it easier to be confident of the treatment.
An advantage of BiPolar (BP) - there IS medication for it. Not with autism, unless you're treating something attached to it (such as ADHD).
We soldier on regardless, but I've always said, it is important to keep an open mind about the diagnosis, because it's always subject to refinement or correction.
Marg