Hiya Italian,
I think in terms of 'right' and 'wrong' it is probably quite 'right' not to start your DS at school in this coming September. It's less than three months from now!
Any further choices, I would say it is a little early for you to be making them now.
You might find, who knows, that by Sept 2015 (a whole year and 3 months from now), he is relatively securely attached to you, and pretty mature for his age, academically capable, and you would regret having to send him to reception; whereas he would easily fit into year 1. Or you might find that by then, some significant SEN have emerged, and what with attachment still being insecure, and him being emotionally immature, he would fit a lot better with a younger cohort. At which point it will probably be easier to convince SENCO etc.
I just think it isn't worth 'fighting' right now, for him being able to start reception in Sept 2015; when it may turn out that actually, by Sept 2015, you'd be happy for him to join Y1. Or, by 2015, you'd be much better armed to actually win that fight.
And others are right, you can review the situation on an ongoing basis. Despite what you think now, come September, you might feel you'd like to try him going to school in the mornings - or, perhaps more likely, you'd like to try him going to nursery school for a couple of mornings. You might still be able to find a place on short notice. And you could always pull him out again if it isn't working out.
Towards the end of the year, you can decide again - try nursery school, try school P/T, stay at home. And come Easter time, you can decide again.
One thing I would try NOT to worry about, is missing out on academic learning. The academic learning that is done in reception, as far as I can gather from reading up on it (DS is starting reception in Sept) is probably about equivalent to 10 minutes one-to-one learning per day, you can EASILY do that at home, without even noticing - and if you don't, it is entirely possible to catch up within a short time.
Why reception is seen as important is not because of the phonics or numbers, it's because the children learn 'how to be at school'. Now in some schools reception really is like proper school - and IMHO 4yo is too young for having to learn that, I feel we do our children a disservice. Most certainly your DS does not need that lesson quite yet.
In other schools reception is very much like nursery school, and can be lots of fun, but then it doesn't achieve this aim of 'preparing for real school' - so those kids who went to reception will be just as 'shocked' upon entering Y1 as your DS would be, having missed out on reception.
Ok, so missing out on reception and starting directly in Y1 sounds like a reasonable choice from my perspective, but there ARE of course downsides (to everything). Such as, joining a class with established friendship groups (though some schools mix up the classes between YR and Y1). I do understand, dear Italian, that you wish to make everything perfect for your DS who has already had enough trouble in his young life. But sadly you can't make everything 'right' - you can't change the fact that he has come home to you a few short months before due to start school, so that he will either have to start school earlier than you would like/than would be good for him, or have to deal with the downsides of starting school at a later time than his classmates-to-be. You can only try to make the best out of the situation. IMO the downsides of missing YR (in part or entirely) are probably much smaller than the downsides of starting too soon. And IF it turns out that going into Y1 in Sept 2015 would just not be right for him, then you would have a strong case for THEN fighting for him to be able to go into reception.
One last thought - if you find your school is not willing to work WITH you on this, not willing to be accommodating to your DS' particular needs - then indeed, maybe this is not the right school for him, not now and not next year either.