Target levels of 5 at the end of KS3 are 'very average' targets. It doesn't sound as though they are encouraging him to make maximum progress possible.
At my school (the one I teach at) for a subject like English we set 1 or 2 sub-levels of progress per year as a target. So for example:
KS2 SATs 4c, end yr7 4a, end yr8 5c, end yr9 5a.
KS2 SATs 5b, end yr7 5a, end yr8 6b, end yr9 7c.
ds1 got 5a for all 3 subjects in KS2 SATs. He has very challenging end of KS3 targets in all subjects - mostly 7s and 8s. He will have to keep working hard throughout to achieve them.
The actual 'progress' can be a bit blippy sometimes though. ds1 got L8 for a Science assessment recently, which is fab. But it's just one area of the science curriculum iyswim, it doesn't mean he could get L8 for "science" now.
But certainly subjects like maths, english and history and so on, you should expect to see steady progress in levels each year.
HTH