I agree with @Kitkatfiend31 If his EHCP specifies 1-1 then he should be in school for the time that he is allocated that support. If it’s full time, the school is funded for that and should be able to provide a support plan full time. If it’s less than full time, there should be a plan for when that support is targeted and what the school will do for any time that 1-1 is not specified.
In any case, any child who starts part time should have a plan for increasing to full time over a short period.
What do the relevant sections of the EHCP say OP? Is it a precise document, as it should be, or does it say things like ‘help to’ or ‘support needed for’.
I can’t remember all the sections, but it would be useful to know what is written.
Knowing they have a child coming into Reception with an EHCP, I would have expected the school to have met with you, particularly his class teacher and the SENCo, to plan exactly what they will do to support your son, what will happen if there’s a problem and how they intend to review his progress.
Simply limiting him to two hours a day, with no plan for increasing that, won’t do.