Hi PeterC....
I would suggest rather than going practice SATs papers - you'd be better off spending this summer working on core skills in short bursts (having been there myself with DD1 between Y3 - Y5 (will be Y6 next year).
Maths: well the advent of computers and video games is ideally suited to teaching maths and learning painlessly.
Great free websites:
Woodland Junior School Maths Zone: resources.woodlands-junior.kent.sch.uk/maths/ - select the area you want to work on and there will be several games to chose from.
If the issue is that your DS has yet to masters times tables (and using those facts inversely to support division) - can I recommend you dowload the free version of TIMEZ ATTACK (link here: www.bigbrainz.com/ - it will test you son to see what level he is at and start from there. He can chose to be a small boy or girl ogre and the free version has two platforms - a dungeon and a castle - you run round solving multiplication problems (which are also shown as multiple additions - thus 4 x 4 - involves your ogre collecting 4 snails and counting up 4 - 8 - 12 - 16) and then a verticle multiplication problem is presented on a wall for you to solve. If that is correct a medium sized ogre comes out and quizzes you on this & the last 2 mutliplication problems you were working on. If you pass that you move on to a new problem if you don't you are crushed by the ogre and start again. After so many levels you get tested by the BIG OGRE who asks lots of questions and notes which ones you are struggling with - you'll get those again on the next level. It's stressful, but great fun and really improves speed. They have a division version (inverse multiplication questions so 36 divided by ? = 9 kind of thing) as well if he's ready.
mumsnet has a link to maths champs - which also has all sorts of games to practice with & is free: www.mathschamps.co.uk/#home - by age he should be playing 9 - 11 games, but you may need to go back to 7 - 9 games for review if there are fundamental weaknesses (maybe doesn't know 5 times table well, etc...).
You can also get workbooks for Y4 level & Y5 level (they vary in difficulty - so have a look in person before buying) from most book shops/ amazon/ newsagents etc... - my advice is go along and have a browse through and see which one he responds to best. Some are full of illustrations & games and others are pages of problems to solve. Let him decide. Set a small achievable target - a page a day - and maybe a routine - do it whilst I'm cooking dinner, getting ready to go in the morning, etc...
Focus on those core maths skills: addition/ subtraction/ multiplication/ division this summer and you'll be well on your way the NC Level 4 next year.
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With literacy there are several things to focus on:
handwriting: is it neat, are letters formed correctly
grammar: does he understand good grammar? (The GCP workbooks are great for this - again Y4/ Y5 workbooks are good review of things he should know - just do a page or two a day - 15 minutes tops).
READING: Every day. Maybe not out loud to you everyday - but talk about it, let him see you reading and have him read with you. Really discuss what's going on in the story, tricky words he may gloss over but doesn't understand, etc...
Visit your local library - they often have reading challenges going over the summer which are really encouraging for getting lots of reading done.
Get worksheets/ pamphlets/ educational colouring books etc... when you are visiting places (museums/ historic houses/ etc...). There's lots of terminology, writing and thinking to do with them and often the child doesn't even realise they're learning.
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My advice is set yourself a small but achievable goal - say 30 minutes a day DC will do a bit on maths & reading. Get them to write letters to favourite TV shows, enter competitions & send postcards over the summer (we even have DDs send cards to teachers). Have him keep a diary (let him chose it from a bookstore/ office supply store). And just see it as pennies in the bank.
In essence by doing between 30 minutes to 1 hour (= 1 or 2 children's tv shows or a long session on an XBox or equivalent) broken up over a day you can build up = 40 hours+ of numeracy/ literacy remedial work over the summer which the school couldn't hope to achieve with your DS over 2 terms (in run up to KS2 SATs).
It's a good routine/ work ethic to develop in him anyway, as most senior schools will have a major increase in homework load as compared to primary school. We've also found that at some point reading becomes pleasurable and you don't have to remind them to read - you suddenly have to tell them it's late, time for bed!
HTH