Hi JustPondering, DD1 goes to special school and is in year 2. She is on P7 for reading, IIRC.
P7 equals 5 National Curriculum (NC) points.
'Good progress' is 3 points per year.
On the link sazale gave, it says:
"Numbers shown in the grid below are National Curriculum Points [NCP]. To calculate the comparison (or difference) between a child?s attainment and their expected level, levels are converted to NCP. For example, a child at the end of Year 4 attaining 4C+ (26 NCP) is expected to attain 3B (21 NCP). Therefore, attainment difference is 26 ? 21 = +5. "
So, if your DS is expected to be a level 1 by the end of year 1 - let's use 1B because that's the mid-point of level 1, that's 10 NC points.
IF he finishes the year on P7, that's 5 NC points.
5-10= -5 points.
That link shows -5 points as being at the high end of '1 year behind'. Just one point more and it would be '2 years behind'.
The difficult part is that progress expected is 3 points per year. That means, that to catch up, your DS has to not only make 3 points progress, but also 'make-up' the 5 points that he's behind. That means that he'd have to be progressing at a rate that is 2â…” times his peers who don't have any difficulties 
As another example, looking at those same charts, DD1 who is in year 2 and on P7:
Expected level at end of year 2: 2B = 15 points
Actual level: P7 = 5 points
5-15 = -10
Well, their pretty chart doesn't go beyond '-8', which is '2 or more years behind'.