I can tell you the 'expected attainment level' for Year 4 but please don't use it to measure your son!
At the end of Year 4, 'average' children usually attain either 3c or 3b with a view to being 4b at the end of year 6.
The average attainment at the end of year 2 is 2b.
I know it must be hard not to measure you son against these targets - however, the real question should be whether he is making progress. What did he achieve at the end of Year 2? You should have his Key Stage 1 SAT results and teacher assessment. If not, ask at his school - even if you are in an area where he is in a junior school they will have had his records passed on.
What level was he working at at the end of Year 3?
'average' progress is about 2 sub-levels during an academic year - however, children do not make the same progress all the time and there do tend to be highs and dips.
Oh and sub-levels are simply something created by schools to divide up the levels to more accurately show progress. To be working at 2a your son has already moved through 2c and 2b and is now working towards 3c.
What interventions is the school using to help him catch up? How much progress is he making and what does the school plan to do to help him next year? Many intervention programmes can help to 'close the gap' by helping the child make more than average progress. As a primary school teacher and SENCo, these are the questions I would be wanting and expecting you to be asking me.