NEED advice!! start statementing process

I wonder if anyone could possibly give me a piece of advice.

My 9 year old son has been diagnosed with ASD, selective mutisms, severe anxiety disorder and sensory integrated disorder. His psychologist has strongly recommended me that I should start statementing process for my son as soon as possible. The problem is that, his school (mainstream) has failed to identify his difficulties/problems, as he is an extremely obedient and quiet pupil and he can’t ask for help. They think he is perfectly OK, as he doesn’t cause problems or complain about anything, although he often releases his anxiety or stress (accumulated in school) after coming back from school, as form of aggressive and violent behaviours. He still needs to wear a nappy for his poo and he can’t clean by himself. He has never done poo in school so they don’t believe it. He often comes back from school with a burse, but he can’t explain why he has got it (he can’t ask for help when he gets injured, he just keeps everything inside him. I also witnessed his classmate injured him but my son even couldn’t tell his friend to stop it or complained about pain)   

 As my understanding, once my request of assessment has been made, LA is going to collect the “evidence” from school within 6 weeks. I am not convinced at all that the school write anything that support my application, as they strongly believe that my son is perfectly capable in school, as he doesn’t complain.

 Anyone has the same experience? Want to apply for statement but school report is not supportive at all?  I really want to start the process but what will happen when school report spoiled my application? (quite likely ended up to a tribunal, isn’t it?)  Any advice will be very much appreciated.

Many thanks!