You are wise to start trying for a statement now.
IPSEA is very good at the whole process of statementing and I would suggest you look at their website for more information - their web address is www.ipsea.org.uk. They have model letters you can use for writing to the LEA in question.
When you write this initial letter you need to write to the Chief Education Officer at the LEA (you can obtain their name from the council) and you also will need to give them six weeks to apply.
Do not let school go anywhere near a Statement application - you as the parents need to do this for him. You have far more power than they do in this regard; school cannot appeal if the LEA say no to assessment. You as his parents though can appeal. The LEA may well refuse your request to have your son assessed so be prepared for that eventuality; you certainly need to appeal their decision if this happens.
If the LEA do agree to assess it can take around six months for the statement to be fully up and running and that is where there are no disagreements on either side. The LEA will seek reports from the professional people your son sees - e.g speech therapist, developmental paediatrician, OT etc.
Our statement application was up and running within nine months but you will need to be persistant with the LEA as they can try and wriggle out of their statutory responsibilities.
You are your son's best - and only - advocates. No-one else is in a better position than you to fight for his educational provision.
Any other questions just ask.