Hi Tiredmama Level 5, is the highest score available (unless your school did the new extension level 6 - not all of them did). It means you should be incredibly happy as he is above average in all three areas. It is also likely, if his secondary school streams, he'll be in the top group.
Roughly (very roughly, can go either slightly up or down depending of subject) 80% of kids reach the expected level 4. Teachers often give their own assessments too and give sub-levels of a, b and c, based on the year. So your ds may have got say 5a in Maths, 5b in reading (comprehension) and 5c in writing. a is the highest sub level.
Many argue, that it's the schools issue and nothing to do with the parents or the kids, because the school is judged accordingly, some going into special measures if too many kids fail to reach target. I however, was quite keen for my ds to get a decent result because of streaming in his new school, where the top groups do Latin, and also, becuase there is a direct correlation between sats results and gcse results. At least then, based on what your child gets, you can think ahead in terms of extra help if need be.
This is my understanding of it all:)
Hope that helps