ASD or PDA - IMPACT?

Our son has just (today) been diagnosed with ASD. He has many behavioral characteristics associated with PDA and I was wondering if anyone has any advice as to what we should do next regarding his diagnosis. If he was to have PDA, this would seem to suggest a different management approach would be necessary within school. He has previously been excluded from school (pre-diagnosis - for persistent defiance!). However has not been back in school full-time since May due to restricted eating/obsessive exercising and a lengthy hospital admission which occured as a result of this (a bit like annorexia but because of rigid thinking/obsession and adherence to routines in the first place). He is scheduled for a phased return to school in September as he is physically much improved but I am concerned about how we ensure he is adequately supported if he is seen to have ASD rather than PDA (if the latter really is the issue). I was told today that we would have one more F/up by the Community Peadiarician, he has been discharged from hospital Eating Disorders Services athough he is still under the care of Community Eating Disorders Services. I'm just not sure who we need to work this out for us so we can put the right support in place for him at school and so that school are aware (school are now much more supportive and aware that he isn't just 'deviant' as his hospitalisation was such a crisis point). Any advice or guidance would be very much appreciated.