Star remember that each school has its own delegated SEN budget, so if dc turns out to need 1 to 1 support on day one )or two_ then they have some money before the EHCP is finalised and funding flows from the LA.
A transition meeting with school and nursery staff, plus you of course, could be a very good thing if the respective teams can make time. And a Transition Plan for how to manage the transition from one to the other.
These days a lot of people know a little bit about autism and assume it applies to every child. So it may be that someone has said that changes are difficult for ASD/C kids and they are thinking that it would be better to have one big change to full time rather than two smaller ones.
They may not be taking into account how tired your dc gets, eg because of the sensory difficulties he has to work with, or because of the endless frustration of getting hi,self understood. If you set out all your observations of dc on a typical day, a good day and a bad day then they may start to get it.
And if nursery can give you some idea of how they manage, and what challenges he faces that may give the new school a better idea of what they face.
How many days does dc do at nursery? Do you think dc can manage half days, or would it need to be less?
Actually, I fear the desire for full time may be finance driven, with the school getting more money for full time pupils. If this is the case you could offer to let him be listed as a full time pupil who may need to go home a lot eg just after the afternoon register, because he is just tired because of his SEN. .
Why would I suggest this approach? because you have a long relationship ahead with this school, and seeing things from their point of view has a value for dc.
You could add that this would give everyone the flexibility that is needed to optimise dc's learning, given that a mainstream class will be so much harder work for him than for his mainstream peers. You can say that it would give the flexibility for dc to stay longer on days when he is able, but to go home if he is overtired.
You know your dc and the situation best 0 but just chucking this idea in, in case it suddenly rings some bells
Good Luck