Pretty much the only thing you can trust about assessments based on NC levels in state schools is that they are likely to be bollocks. Same goes for what they say about rates of progress.
The likelihood that the levels you've been given are bollocks jumps to near-certainty if the school is primary, if it has recently been in special measures, or if it has been rated outstanding by Ofsted. The assessment system schools use is wide open to abuse, and every incentive is there for schools to abuse it.
Do you just want to check DS's progress for your own piece of mind, or do you need independent evidence to support a case? Either way, there are tests out there that are much more rigorous and less prone to gaming than teacher assessment - but if it's evidence you're after, then you'll need a professional to run them and assess them.
Here's an example - DS1 has a pretty severe language disorder, dx'd by four separate professionals. At the end of Y6, independent testing showed that he had a reading age of 6yrs 5 months (Edinburgh Reading Test), and an understanding of grammar roughly equivalent to that of a pre-schooler (TROG-2). According to these independent tests, he was 5 years behind his peers in most areas, and further behind in some.
According to the school's data though, everything was going swimmingly. DS1's school gave him a Level 4 for reading in his Y6 SATs, after apparently making a metronomic 2 sub-levels of progress each year. Their data was complete and utter cock, every last fabricated bit of it.