yanbu to be concerned but the fact you are AWARE is great, and means you can help your child :)
if it helps any my ds1 is a late aug bday and ds2 is july, so both due to start school just after their 4th bdays, there was no way ds1 was ready so we didnt send him, ditto ds2 and i home-schooled until ds1 was 9 and ds2 was 6. anyway they are now in yr 8 and yr 5 and both top of their yr group and doing brilliantly, you would never know they were the youngest ones in their yr.
ds2 has had a few issues with maturity but in the last 6mths has come on really well with this.
the schools are aware that some children are younger and they dont all learn at the same speed etc and most good schools will take this into account and work with you and your child :)
my ds3 is dec born and did fine starting when 4 and 3/4, tho still struggled with reading etc but it has all clicked into place in yr 2, which is quite common i have found.
my ds4 is march and he starts this sept, he is desperate to go to school and is ready imo, but he has 3 elder brothers and i think that really helps! he wants to do whatever they are doing and strives to compete with the and does so!
all children are different basically but the school should work with that.
talk to the school, reassure your child etc, but imo the first 2 yrs of school are really all about the socialising, learning the rules, how to interact with others, work in a group etc and yes the basics of reading and writing but i wouldnt focus too much on reading etc if he is behind in reception or yr 1. what is important is that he ENJOYS school and loves learning and he will get there with the rest :)