HELP! 5 year old has a 'Short CDAC: assessment next week,.. What to expect.

Hi all looking for advice please. My son aged 5 was diagnosed with ADHD this year and Sensory Processing Disorder as well as Tics. My son had an ADOS test done 6 weeks ago, which came back as possible HFA/Aspergers. Have had a letter through stating that he qualifies for the shorter CDAC assessment, and we will be there for 1 1/2 hours next week, where I will know whether or not my son will get a diagnosis. Can I ask how a short CDAC differs from the general CDAC which can usually take 4 hours? Is that because my sons condition is quite noticeable? Just wondering if anyone can give me any pointers as to what will happen on the day as of course my sons not having a full assessment, and a shorter version so would like to prepare him as much as I can as he is no good with change.