If I get told a student got a level 5 at primary, I would wait until their first assessment with me to see where they really are. Mostly, they come out a high 3/low 4, which is more realistic for secondary as we expect them to deal with harder concepts than at primary.
I teach humanities, so I was always more concerned with their reading age, as this gave me a better indication of how they would do. Someone with a GLA of -7.4 would find it well nigh impossible to access the curriculum, whereas someone with a GLA of 13-14 at age 11 would do well.
I think SATS are a pernicious waste of time and energy and tell us nothing apart from how a child performed on a particular day. They tell me nothing about how they learn, how they cope in class or what they'll get a GCSE. They are a stick with which to beat teachers, and take attention away from the wider curriculum that should be taught at primary.
End of rant! Honestly, don't stress yourself or your lad about SATs, it's pointless. As an exercise in how to approach an exam they're useful, apart from that, they're not.