Can you get the school to identify where he is behind?
Did you get a report at the end of last school year which rated him on about 8 areas, split into 3 areas each (inc maths, literacy, physical, social, communication). Do the two sets of information agree, and does it match with your view of your child?
Are you prepared to say what he CAN do? There are loads of people here who can let you know where you might be able to help him - and not just the people who say "My Y1 child can do quantum physics, write a novel, and read war and peace".
You son is lucky enough to have a Mum who cares about him, and reads to him - those are 2 of the markers for a successful journey through primary school.
If he isn't that far behind, and you possible can, I think I'd keep him with his year group. If he is that far behind, would the school help you get a statement to gain him additional support?
Hope you find a solution you are all comfortable with.