An EHCP application will require evidence from an educational psychologist, SaLT etc as well as the school. They will assess your son independently, discussing his needs/ difficulties with the school but also with yourself. It’s also worth considering that the reports will often focus heavily on difficulties rather than strengths and reflect your son on his worst days, so if sometimes he can join 6 words together but on a bad day can only use single words and has meltdowns then that is what the report is likely to state.
It will be the same when assessing him against the EYFS outcomes, skills have to be well established so if your son is able to display counting skills at home within familiar activities but can’t transfer them to a new counting activity in school then he will be assessed as not having achieved those skills, even if in specific situations you have seen them.
It sounds like your son would benefit from extra help and all an EHCP will do is outline exactly what that help should look like and what the school needs to do to ensure it is provided. I would go along with the assessment, if the school are exaggerating his needs then this will be reflected in both your views and the SaLT/ Ed. psych reports etc and it’s unlikely the plan will be issued. If an EHCP is issued then it will ensure he gets the help he’s entitled to.
Also, the EHCP will not necessarily last until he’s 25 - it will be reviewed annually in a meeting with yourself and all professionals and if his needs have changed then the EHCP can be updated to reflect them or it can be removed if it’s no longer required.
Early intervention is key on supporting children with additional needs so getting him the right support now is likely to decrease the likelihood of him needing an EHCP in the future rather than increase it