You have to see this from the LA's eyes, Deadly.
The bar for a Statutory Assessment is high. The LA has to believe that a Statement of SEN is 'likely to be necessary'.
The need for a Statement is down, essentially to one criteria:
The school cannot be expected to make the necessary provision out of its own resources.
There are different ways to fulfil that criteria, however. In a nutshell, the criteria is going to be fulfilled if:
-He won't make adequate progress even on School Action Plus
or
-He will disrupt the education of other children
or
-He will cause other children to be at risk
or
-He will be at significant risk which cannot be avoided himself
or
-He needs expensive specialist equipment
or
-He needs regular specialist teaching
or
-His needs are severe and complex, requiring multi-agency collaboration
etc.
So, what you are trying to do is give them a concise, clear document which they can 'tick boxes' with.
You need to use phrases that imply a high level of need, and a high level of risk without the right provision.
Use words such as:
Significant
risk
harm
disruption
failure
specialist
complex
severe
etc.
I don't think you need to have a crystal clear plan for his educational requirements at this stage. If you knew what he needed you wouldn't need assessment. However, you do need to demonstrate that he has a high level of need.
I think you need to break it down into areas of need, and address the need for SA with each. Concise but specific.
Do make sure that you include things that are relevant to educational issues. So, if he has difficulties breaking routine, you need to highlight the difficulties he would have with transistion times and that he needs adult support to reduce his anxiety at these times. Also, what the consequence is both for him and the class as a whole if that doesn't happen.
Overall, a relatively passive child has to be shouted about more, because if the only effect is harm to that child, they will slip under the radar. A child who will disrupt the whole class day in day out is going to get a statement much easier, because it causes trouble for everyone, IME.