I think it would be worth finding out where this 2a in reading has come from. Ideally, it will be a solid teacher assessment, backed up by tons of evidence drawn from throughout the year and supported by one test (perhaps the optional y3 SAT), as is the case now with y2 end of year teacher assessment.
Y2 SAT results are a small part of the teacher assessment in y2, and would probably not be passed up to the y3 teacher.
The plateau effect used to exist because the y2 SAT mark was the only result that stood, then the y3 optional SAT mark would be used to give a level in Y3 - Y3 tests are timed and Y2 tests aren't, so of course lots of children appeared to get the same result two years running. Now schools have to use their own assessment policies which should be the same throughout the school - a 2a in Y2 should be exactly the same as a 2a in Y3.
Good teacher assessment is now statutory in Y2, and most schools have caught up with this kind of procedure in assessing other years. Some schools assess using only one test result, and this is poor assessment.
I suspect that the 2a comes from the Y3 optional SAT - as I said, this is timed, so the result when built onto y2 results could be the same. However, this should be backed up by the teacher's own assessment, her view of how he is progressing. And if it isn't, and he hasn't made any progress - why not? Either situation is worrying and warrants an explanation!