Coogar It is best to apply for a statutory assessment yourself.
Personally, I would wait until after the assessment in Feb to apply or tell anyone. I'm a suspcious thing and would want to know that there had been no pressure on the assessor to NOT diagnose. This is of course very unlikely but I do know of one case where this happened.
However, there is nothing to stop you from preparing the letter to be in a position to send off the application the day of the dx (if you get one).
It would be helpful too, at this stage to enter into written correspondance (email will do) with the school about their concerns, as well as send an informal note after every chat with them to clarify you understood what they are saying.
For example, you should not need a dx to get extra help with your learning. You should need extra help with your learning to get extra help with your learning.
So you might want to ask the school what he needs help with, so that you can try and work on some of those things at home. This will give you information to be able to help him, but also information for your application of SA, as well as 'evidence' if you need it to negotiate etc.
With the exception of holding off telling them you will be applying until after the dx, do try as much as is possible to have a good communicative relationship with the school. However, don't do this at the expense of your ds' education if it looks like it might go that way.