Mrs C24:
first off have a look at Mumsnet pages of expected progress for the 'notional typical child' through NC Levels here: www.mumsnet.com/learning/assessment/progress-through-national-curriculum-levels -
So they have as END Y4 achievement at 3b - which is in fact where your son is for Reading & Maths. So he's about where he should be. He's a little below where he should be for writing.
With writing - there's kind of a progression of aspects that teachers are looking for - and these VCOP pyramids really help to lay out that sequence of development for vocabulary/ connectives/ openers (ways to start a sentence) and punctuation: displays.tpet.co.uk/?resource=387#/ViewResource/id387 - these are laid out so that items at the top of the pyramid are basic skills and things at the bottom of the pyramid are more advanced skills.
How you can encourage writing over the summer and get in that bit of practice:
postcards - have your DC send postcards from places you visit (even if only an outing into town).
Thank you cards - have your DC send thank you notes for presents/ treats/ nice days out (maybe make some for his teacher/ TA now!)
Fan letters: Write to your favourite author/ tv show. Some books have prizes if you write in. DD1 did this in Y3 when she was mad about Rainbow Magic Fairies - the prize draw was already over but the lovely people there sent her tons of stuff - a T-shirt, pencils, stickers, bag, letter from the fairy, etc... She was over the moon. [She'd written to say this was the first chapter book she'd ever read in a very sweet little letter full of punctuation/ spelling errors I let go - I wrote to them afterward and thanked them for encouraging her].
Cartoons: encourage your DC to make a cartoon and fill in speech bubbles.
Keep a diary: Admittedly this may be more popular with girls - but buy a nice notebook & pen and encourage them to write stories/ poems/ journal.
With maths - take an inventory. Does your DS know his times tables to x 12 - I mean really know them if you turned to him right now and said what is 8 x 7 could he answer instantly and then if you fired at him what is 72 divided by 8 would he know it was 9. If he is not totally secure on that summer is a great chance to secure those skills and there are lots of free games on-line to use:
I'd start with Woodlands Junior School Maths Zone: resources.woodlands-junior.kent.sch.uk/maths/ - just select multiplication or division.
Mumsnet advertised Maths Champs at some point - the games are laid out by age 5- 7 / 7 - 9/ 9 - 11 - which isn't ideal as the times tables are spread across all 3 - but the games are really good: www.mathschamps.co.uk/#home
There are a lot of games/ resources on multiplication dot com: www.multiplication.com/ - my DD1 liked Sketch's World and Ping Pong the Panda - but there's tons to choose from.
If your school belongs to My Maths there are tons of games on there - try snaky sums to improve multiplication skills (low stress - no issues if you get it wrong - but the game speeds up).
If you child likes a bit of stress - try Timez Attack (you can download a free version with 2 platforms - dungeon/ castle - which we found was fine for us). The programme will test your child and start from where they're at. You're cast as an ogre and run through the dungeon/ castle solving multiplication problems which are presented as traditional adding up - 3 x 4 is counted up 4/ 8/ 12 and then the vertical problem is presented and you type in 12. Every few of these a medium-sized ogre quizzes you on what you've been doing. And after so many 'levels' you are quizzed by a giant ogre. Things you get consistently wrong appear again on the next levels. There also is a division version. www.bigbrainz.com/
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For reading well I would advise that you just keep him reading over the summer. It doesn't necessarily have to be 'proper books' - magazines/ comic books are fine too. But encourage him to read to you a few times a week so that you're hearing how well he's sounding out words, discussing words you feel he might not understand and discussing the story (why do you think that character might be lying?/ who do you think stole that object? etc...).
Many libraries have activities - including awards for reading so many books over the summer - which may be fun for him to join. Get him to read a book before seeing the film this year. Cressida Cowell's How to Train your Dragon books are great - don't just watch the cartoons/ movies.
Read one book that is a childhood favourite with your son. DH has read The Hobbit and The Weirdstone of Brisingamen to our girls. I've read Pippi Longstockings & Charlotte's Web to them (a bit earlier - and admittedly a bit girly). But something like Robert Louis Stevenson's Treasure Island or Kidnapped - are great stories and listening to you read and just absorbing the language/ discussing new vocabulary is really beneficial.
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Your son is doing o.k. - not great, but o.k. So it's up to you of course - you may feel strongly this is fine or you may feel he should be doing better (as you said Year 3 wasn't very productive) but during the summer holiday if you just do a little each day - 15 minutes reading/ 15 minutes maths computer games and maybe now and then sending a letter. We sacrifice a bit of tv watching for this kind of thing and do a bit while one or the other is having their bath in the evening - but otherwise enjoy our summer fully.