Feel for you Kay - my 6 year old has just been excluded from school also.
The board of governers should be your port of call. You should have got written notice from the head stating why and when the exclusion includes, and it should also contain the details for contact.
Ask for a copy of the full written report that HAS to be made for the incident.
This willl stay on his school record, if you disagree with the exclusion, make your complaints and arguements.
And while you are at it, ask for a copy of his school report. If good behavior is consistent, you can ask for an inclusion of ch