Primary School- EHCP

Hi all. My son is 4 and starts school in September 2021. Does anyone have experience of Early Educational Healthcare Plans? I understand that this is a legally binding document that states what support schools need to put in place, in addition to the SEND support all schools have as standard. I think my son would benefit from having an EHCP (he is 26 months behind in all areas asides from gross motor skills) but my local authority's Senco Coordinator does not think he meets the criteria. Unfortunately I don't feel that her guidance is based on my son's best interests so I seeking to understand what really determines whether a child gets an EHCP.