Bonkerz, the dx shouldn't be the only pathway to more help. I say shouldn't because I know it isn't always like that, but J was Statemented even before he had any dx and got DLA even before he had any dx. The only thing that the ADHD and autism diagnoses did was give more of a description of his difficulties.
The only place that needed a label to give us some help were Social Services - but the ADHD dx, which you have, should be sufficient for them.
I know that a label can open up more things e.g. autism training for schools etc. but in terms of core resources, as flyingmum says, there are clear needs here and the Statement and the support shouldn't be dependent on an ASD dx so an inaccurate dx wouldn't really move anything forward, would it?...unless I've missed something? - which is very possible!
I even had a meeting with a mediation person recently where she repeatedly (and patronisingly) reminded me that the new dx probably wouldn't affect the LEA's decision as decisions were based on need not dx think I knew that already!
That IS the way I guess things have to be - especially when dxs are so hard for some to get because of waiting lists etc. So I guess what I am trying to say is, don't worry: he has very clear needs and support for those needs should not depend on an official ASD dx, so don't stress about dx!
Sorry, really tired so this probably doesn't make sense!