You can also say, that although school is supporting him now at primary, at secondary the demands on him will be far greater and he will need considerably more support.
Ask SENCO what support they offer for LAC, are they aware of some of the issues that might come up? Like family trees ( a common topic in Year 7) Do they have a zero tolerance of bad behaviour - how will they respond to him misbehaving, will it escalate if they shame or punish him? Is there any pupil premium for him and how is going to be used?
The school will need extra money to put in specialist teaching for literacy, for nurture, for social communication skills, for 1:1 support sometimes, extra pastoral care. Not all of this will be found in the existing budget. The school will be stretched without extra from the LA. With a Plan he will already get a certain amount, without school having to prove anything, but they will need more, most certainly if your sons needs are going to be properly met. At least this is what SENCO in a school confessed to me..they are making cuts all the time in the SEN budget,whatever the PLAN says and it is not legally enforceable as you say. However, an EHCP without extra money attached to it is pretty useless too, (which is the way the LA wriggle out of their responsiblities)
I was told my child would not get an EHCP on the same grounds as yours, not far enough behind. It did not stand up to the EHCP process and despite being refused initially, he was eventually assessed and given one. And my son was only about I year behind, and achieved a 4 in his Year 6 Sats. He has ASD and dyslexia/dyspraxia. His behaviour was good. Still has an EHCP for social communication and dyslexia/dyspraxia.