Thank you for that information. Even by Local Authority timescales the time that this diagnosis took is appalling.
If it can be shown that there was a failure to diagnose and or/ act promptly, and this has had a detrimental effect on your son, then there could well be a case for negligence.
A solicitor would review the information you have on file, and ascertain whether there is a good chance of succeeding. If the chances are better than 50%, then they could take the case on a conditional fee (essentially no win no fee) agreement, to save costly fees from the outset. A medical expert report would be commissioned and then negotiations would begin with the LA/ NSFT which would normally resulting an out of court settlement for compensation.
These type of cases/ settlements are often protected by non disclosure agreements though.
As to a way to make a change in policy, this is more effectively achieved through campaigning, lobbying, involving your MP, petitions, local press, that sort of thing. If the local press did pick up on the story, that could give it some momentum. This is very different though from pursuing a legal case and the two may actually be at cross purposes with each other due to the confidentiality of out of court settlements. You could, if you were looking at this route, look at crowdfunding as another way to raise the profile, for example to pay for private tuition for your son who has missed out on so much schooling as a result of the failure to act and diagnose properly.
I hope that this assists? We can't take a case on for you here, so I can only offer general advice at this stage.
Can I clarify anything further today?