Please ! I'm having my lunch here !
I don't think you need to worry just yet, Bananas . What weak areas have you highlighted ? and are you sure they are weak, rather then just areas he's not as good as ( yet ) if you know what I mean ? I find with all of mine when they were learning something new they seemed to forget other stuff , but then eventually it all came together.
Now, I'm sure in your library of workbooks you can find a year 3 book which will cover all the topics if we are talking about maths. Has his teacher said he needs to work on anything ? I know you are in a highly pressurised area , but I found with my 2 boys everything came together in year 4 or even year 5 and very quickly too .You might find the same . I wouldn't worry about topics he hasn't covered ....you might find he packs his spotted red hanky and leaves home ! Maybe some online games to keep it light ? ( sorry Yoyo..did I get that one in before you
) With the maths it is the same old things they are learning i.e fractions , multiplication, it just gets harder , so the yearly repetition will help.
To answer your question about organising with my DS when they were mid way through year 4 I started going through the maths topics from a key stage 2 book and tying them to a chair for spellings etc which they hated. Hope that helps !