I'm not sure anyone understands it, including (often) schools and local authorities, which is a big part of the problem. I know of a couple of cases which were rejected recently for the same reasons, but which succeed on appeal.
Is your school SENCO any good? Your local authority will be able to provide details of the free independent supporters in your area and they should be able to help you get together the right kinds of reports. Certainly thorough assessments from school and paediatrician showing where he is and his difficulties in progressing even with SEND support at school (they would need to outline clearly what they've been doing so far). There are also several charities that offer support relating to dyspraxia, which might be able to advise on the right kind of evidence to gather. I think it sometimes helps to get independent reports, but obviously that can be prohibitively expensive - though again the charities may be able to advise.
It seems to be becoming more common for children to be refused EHCPs due to lack of evidence when in fact the evidence is precisely the kind that should be gathered during the process.
Given his age, a review for transfer into secondary school might provide a good opportunity to ensure he gets the right support moving forward.
Hope you're bearing up.