Disagree entirely with the others. My DD2 and Ds were similar ages when they started in Reception, but far younger when they started school - only just over 3 - as their school has a nursery class.
Reception is NOT just about learning letters and numbers - it's about learning how to socialise, how to sit still and concentrate, how to join in with other kids...all things that your ds is ready for but needs to learn by being in that situation. If he starts a term late or later, he will simply be an extra term or more behind the other kids, making it even harder for him to settle in successfully.
Re letter/number skills, ay half-way decent teacher will know that these skills vary hugely at that age and will provide differentiated learning, so that the same class can happily accomodate both children who barely recognise (let alone can write) a letter, and others who are easily reading and writing.
Of course it's more stretching for the younger ones, but that is a huge advantage, in my opinion.
My DD1 was one of the oldest in herclass, and has always been at a disadvantage as a result - the work has always been too easy.
Both of my younger two have loved being among the youngest in their classes. My DS was only a tiny bit older than yours when he started in reception this year, and his reading and writing skills were correspongingly low. But he'slearnt loads, is having a brilliant time, and I cannot imagine how cruel it would have been to have deprived him of the opportunity to make this progress, just because I believed he wouldn't be up to it.
Your son has months to make progress in - and I doubt he is uniquely dim or immature. He will be within the normal range for the school, which is why the school thinks - quite reasonably - that you are over-reacting by suggesting he will struggle.
Have more confidence in your child - it is precisely these low parental expectations that the child picks up on, and which duly become self-fulfilling.
Your ds will be fine - unless you've chosen a sink school, any half-way decent school should be able to accomodate his needs just fine.