From the angle I am looking at it your DS is having a real hard deal. He is struggling and he shouldnt be left in this state. (by the school I mean :))
He may hate assembly I know my ds did and would only settle if he was left on his own in the class first.
EPs cant diagnose anything, they can suggest behaviour strategies or could hint at something from past experiences of assessing other children.
Your DS is being excluded because he is unhappy. Either he just wants out and this is the way he can get home or he is trying to tell them he doesnt like it there and cant cope. Read between the lines rather than look at the letter showing assault. What you are really interested in is WHY he is hitting out.
Keep a diary going of everything and anything you find concerning. One for home and one for school issues. Tell school you are taking this very seriously indeed and is it possible for a home/school book to be started as you are seeking further proffesional help and the more records of what is going on the better understanding of his needs will form.
If an EP has been called in then he must be on School Action Plus? which is one step away from a statement request (I know things are changing but at present this stands)
I would in all honesty apply for a statement yourself. This is nothing to do with "labeling" at all. Its easy to apply and the site called IPSEA has a template to start you off and we can all help here with the rest. :)
In the meatime take a visit to your GP and show him all the letters, show him the EP report when it comes and tell him you have serious concerns about his development and would like to rule things out to put your mind at ease by having a referral to a developmental peadiatrician.
This will do no harm as if there are no concerns and you believe that they will tell you if not they will refer you to someone who can further assess and stop this school ignoring your sons needs and just excluding him because they dont know what to do next.
Sorry this sounds dictatorial but I would go by your own instincts than follow a schools any day x