How did he do in his Year 2 SATS. I would think that you certainly have a right to know that one.
As a rule of thumb a child is expected to move 2/3 of a level each academic year so taking 3c as a starting point for KS1 SATS (which would not be unreasonable for a prospective 11+ candidate), expected progress through the curriculum would be as follows:
End of Year 3 - Level 3a,
End of Year 4 - Level 4b
End of Year 5 - Level 5c
KS2 SATS - Level 5a
Try writing and asking nicely for a meeting with the headteacher to discuss his attainment scores.
If that doesn't work come back to the forum, can advise of other ways of requesting information but not generally recommended unless you don't mind upsetting the school