The old system that is currently being phased out involved measuring children's attainment levels at the end of KS1 (i.e. about April in year 2) and then measuring them again at the end of KS2 ("the SATS").
All children were expected to make at least 2 whole levels of progress per key stage, and the school / individual teachers were held to account if that wasn't achieved. Attaining more than 2 levels was great, but there were no rewards for that as a teacher in the way that there was approbation for failing to meet the target for all children.
Obviously very able children could easily exceed the target and the least able needed a lot of coaching to get them to it - they call it intervention.
The system led to some manipulation, with an artifically low KS1 score obviously helping the KS2 teachers meet their targets without having to set special work or prepare lessons purely for the benefit of one or two very able children.
One of the obvious tricks was to fail to push the most able on during KS1. Maybe a child knows every part of the syllabus up to and including all the elements of GCSE. However, if they don't demonstrate each part in class to the teacher on three separate occasions, then the child is deemed not to know it. So, if a KS1 teacher doesn't provide an exceptionally able child with as many opportunities as he/she could easily cope with to learn new things and doesn't give them the chance to show her what they can really do, then as far as the teacher and the school are concerned, the child cannot do it. i.e. No evidence = its not true.
The mother/ father telling the school that their child can do these things is met with agreement that their child is very able but their child is not evidencing their higher level of attainment in class and until they do, they won't be ready for the next level of complexity. The parent (i.e. me) then points out that the teacher isn't providing the opportunity for the child to demonstrate the new level of attainment and the teacher replies that she regularly tests but that she'll make sure your child is included in the next batch of children that she questions to see if they have reached their current targets.
Three weeks later, she tests, sees that the current microlevel has indeed been reached and sets the child with a target to reach the next micro level. Meanwhile the parent walks away from the meeting feeling like the teacher thinks they are pushy parents, and possibly deluded too. Moreover, they are aware that if the objective was to get their child taught at an appropriate level, then the meeting was a complete failure. (All of which obviously put you off going back to see the teacher again unless you really have to).
And so on it goes until your child is given a level 3 test paper at the end of KS1 which they can magically do. Everyone is pleased. The KS1 teachers have gone above and beyond for this child bringing them up not to 2, but 3 levels during KS1. The KS2 teachers are now automatically set a target of getting the child to level 5 over the next four years.
With the introduction of level 6 papers, things improved a little for very able children. The papers are optional but most schools choose to send a few pupils for them. Children like your son (and mine who is currently in year 6) are obvious candidates, but its hard to say where they really would have got to if they had been allowed to learn without restraint.
Sorry.. very, very long post/ rant!