"The letter that arrived today says they now have 16 weeks to prove if the school will be able to look after him under an IEP and will contact his doctors etc."
I sincerely hope that's your paraphrasing of the letter???
The letter should say that they have decided that it's necessary to undertake a Statutory Assessment and that they will be contacting all the relevant professionals. It should also invite you to submit your parental assessment.
They should not be suggesting that they should be proving that your DS doesn't need a Statement - they should be working from the assumption at this stage that he probably does need a Statement, because that's the basis for agreeing to Statutory Assessment.
I don't really understand where you are in the process, mostly because there isn't any stage in the statutory assessment process that allows for 16 weeks. Is this a typo? The LA normally has 6 weeks to decide whether to assess, then 10 weeks to do the assessment. At the end of the first 6 weeks, they must tell you whether they've decided to carry on to do an assessment, and if they decide on assessment, then at the end of the 10 weeks they have to let you know whether they've decided to go on to a statement, and they must produce a proposed statement within 2 weeks. If they decide against assessment, or if they decide to assess but then decide not to issue a statement, they must give you notice of your right of appeal.
So we have passed the first stage. The letter that arrived today says they now have 16 weeks to prove if the school will be able to look after him under an IEP and will contact his doctors etc. At the last meeting everyone was in agreement to attend mainstream school he will need a FT 1:1.
So does this mean we still wont know until after these 16 weeks? Are they likely to turn it down the first time?