I completely understand how vulnerable you feel, it is really no wonder. Nobody should go through this kind of thing in this country, in 2012. Particularly not your son.
What has happened is that they don't understand who you really are and what has happened to your son. It's too complicated for them and so they want you to just go away. All of these people are public servants and they are employed by you. Serving the public is at the heart of what they do and if it was a little less complicated and resources weren't so tight they would probably be serving you and your son properly. It's obviously too much of a headache for them. The sentence about you having 'grievances' is the thing that they are wary of. They are hoping you will just go away.
Regarding press confidentiality - it is taken very seriously and your identity will be hidden if you request that. However I would use press exposure as a threat if you need to rather than an actual weapon. However on second thoughts think that finding an advocate or an ally might be a better solution now.
If they have an opinion of you that is fixed, that is what needs to change. One thing to help this is if you you write a long statement explaining everything that has happened, very clearly perhaps with a summary of key dates of what was said when. Make sure all the agencies get a copy, and make sure that on the letterhead, you state that each of them are getting that copy. That way they know it's not personal, or that you are getting 'delusions' about one agency or worker.
You could also find yourself an advocate. There are agencies that provide advocacy and that may be useful, if not it would certainly help to have an ally, a friend or family member that can be by your side when you speak to the school etc. As I said, it is their perception of you that needs to change and you will need to do that for them.
Regarding discrimination, when the police told you that the CPS wouldn't take it any further because of his disabilities, that's where they are discriminating.
Please bear in mind that I'm not a professional but a parent of a child with SEN, so run any of this by your solicitor first to make sure you're not creating more problems. The letter you should write anyway, but perhaps get someone to look at it first. Copying the letter is the most important bit - the agencies need to know that you are sharing information about them - it keeps them on their toes.