Hi user, I think it is quite usual for children to arrive from other countries and be 'behind' - as barely any other country starts formal schooling as early as England! So one thing to reassure you, the school will have experience with this kind of situation. Your son has the advantage of at least knowing the language - many children arrive not speaking English on top of not having started school yet.
Another thing to reassure you: In my opinion, Year One is very very tricky to get right in schools here, and for many children it is a bit of a waste of time. I know this will be controversial. I have formed this opinion from my own, very able child's experience in Y1 and from volunteering in a Y1 classroom. The range of abilities - and the range of how ready the children are to do 'formal work' - is huge in Y1. Very able children are often let down because in contrast to the play based, often child-led reception year, now they have to sit and complete worksheets that are too easy for them, and don't have time or opportunity to follow their interests and discover things by themselves, by play, as they IMO should be doing at that age. And very many children in Y1, be they able or academically struggling, find the 'formal work' thing hard. More often than I'm happy with I have seen a child put in front of a worksheet a) not knowing what they are meant to do and b) even if they did know, they did not have the skills necessary to access that kind of work. So they sit and wait until someone gets round to helping them. This sitting and waiting is the reality for many Y1 children and it makes them bored, lose confidence, turns them off from school and learning. It's not just a waste of their time (they should be playing and learning through play), it is positively counterproductive. I know some schools/teachers do better than this, but I hear a lot about schools that are very similar to this. And with the further cuts in education that are coming, there will be even less funding for 1-1 support and such. For my second child I am seriously considering sending her to school for reception, then HE for Y1 (and perhaps Y2, depending on how she is developmentally) before sending her back to school when she is developmentally at the stage where she is ready to learn in a school context.
So what I'm saying is, do not worry too much about skipping Y1! See it this way: While many/some of your child's future classmates are, right now, sitting at desks waiting for someone to come round to help them, or sitting at desks waiting for something interesting to happen because the learning objectives they are supposed to be working towards are things they knew how to do a year ago, and learning the sad lesson that school is boring or that they are obviously 'too stupid for school' - your child is, developmentally appropriately, playing, exploring, following their own interests, and probably learning more than you can tell.
Your son will be 6 1/2 when he starts formal schooling and that is IMO a great age for it. He will find learning to sit still and listen and do 'formal' work so much easier than most 5 year olds do. And unsurprisingly, average 6.5 year olds are capable of learning to read, write and do sums much faster than average 4 or 5 year olds. Chances are he will catch up in no time at all, and may go on to outperform many of his peers as he had that crucial extra year of child-led, play based learning that English children are cheated out of.