It is brilliant the school is on the ball with seeking EP advice.
For the EP assessment, it can be helpful to make a timeline of events/any previous assessments etc. Also think about making notes on things like developmental history, needs/difficulties, likes/dislikes/what makes DS happy, what support is already in place, what has previously been tried, what worked/didn’t work, what is needed but unable to be delivered, what is important to DS/you and what is important for DS, how DS communicates/how others communicate with him, aspirations, what you think DS needs, a bit about family/any other important people in DS’s life.
It can also help to write a list of questions to ask the EP such as what assessments will be undertaken, what observations will be done (e.g. structured and unstructured times, with peers, with adults), will they meet with DS 1:1, how long they will spend with DS. Ask if the EP will make recommendations for the need for further assessments such as OT where they believe they are necessary - sometimes they do and sometimes they don’t.
What support is the school already providing? What have they tried that hasn’t worked?
Have you tried some PDA techniques?
Has DS had a sensory OT assessment?
The EP won’t diagnose ADHD.