As he's five its a very good time to put your request in for the statement now.
You are correct; just get the initial letter sent in, Not that you would but certainly don't leave this statementing business till he's in year 3 (Junior school). The longer you leave it the harder it seemingly becomes to obtain a Statement.
Would suggest you use the wording from the initial letters that IPSEA use. These are good and importantly to the point.
Bear in mind as well that these things take months to set up, average is six months and it can take longer than that. So you're looking at his statement starting in year one if you're fortunate.
SALT in a statement will be a challenge for you to receive, it will need to be in Part 2 of this document.
Some LEA's are now actively devolving powers to school basically saying to schools, " well you sort it out". IPSEA on their website have more details re devolving of LEA powers to schools. This causes no end of problems both for school and SEN children as without a statement these children have no legal protection.
You don't need much initially - what you need is the initial enquiry letter to the LEA, the LEA must reply within six weeks. Note that date on your calendar. The initial task is to get the LEA to agree to assess. Bring your big guns in when you need to in the (not unfortunately unlikely) event this goes to a SENDIST tribunal; this will save you both time and expense.
Keep trying IPSEA's helpline; I have been able to previously get through to them late morning and they also have an email facility.
Also NAS or SOS:SEN can give pointers.
HTH, good luck!!