I'm a teacher, and England's very strict year group rules really annoy me! In most infant classes the children in the lowest ability group are generally the youngest in the class. They struggle as they work their way through the school, and in some cases never catch up. Those that do catch up have often had to cope with years of finding things difficult, whereas if they had been allowed to start school a year later all of that could have been avoided as they would have been much more ready for the work they were set. Unfortunately, in my experience it is very, very, very rare for a child to get held back a year, even when it is patently the best thing for him/her. As far as I know it is only done on the recommendation of an Educational Psychologist, so it's good that you're getting a referral to one as a first step. Even then it's unusual. I once knew a boy with a 2 letter name who could not write his name (or any letters of the alphabet) when he went into Year 1 , and could not count past 2. He also had other difficulties such as if he started to walk across the room and a chair was in his way he would not be able to move around the chair, but would just keep walking forwards, oblivious to the chair scraping across the floor. He was a summer birthday, and had suffered some neglect in his home life before he started school, and so evidently could have done with repeating Reception, yet he was not allowed to. I do know of a boy who was allowed to repeat Reception and then remain a year beind his peers. He was an August birthday, and 3 months premature, so if he had been born when he was meant to be he would have been a year below anyway. He was Statemented, and at 11 went to a special school. However, even then, once he went to the special school, even though at primary he had just finished Year 5, he was placed into Year 7, to rejoin his original year group.
It's such a shame that holding children back a year isn't done more often, particularly for the very youngest children in a year group. It seems so silly to me. Why should a child be in a different year group, doing much harder work, than a child born only a couple of days after him or her? That seems mad!
Anyway, sorry not to have more constructive advice. As far as I know, it is unlikely your son will be moved back a year, but it is possible with backing from an Ed Psych. If you can afford it, it might be worthwhile to get a tutor for your son for an hour or 2 a week. My mum was a teacher for many years, and now she is retired does some tutoring, and is always told by the children's parents how much confidence she has given their children. She also brings them on academically in leaps and bounds. One boy, who she began tutoring a year ago aged 10,didn't register on her reading test (which only registers scores of over 6 years). Now he has a reading age of 6 years 10 months. Not brilliant, obviously, but in 6 years of school he had made virtually no progress in reading, whereas now, after just a year of working 1:1 with her, he can read and understand simple books. I'm sure he would be a lot further forward if she had started teaching him earlier, so now is probably a good time for your son to start having some individual tuition. Good luck with everything!