Hi SunshineMK
first off take a look at MN Learning pages on progress through NC Levels here: www.mumsnet.com/learning/assessment/progress-through-national-curriculum-levels - scroll down and you'll see from the table at the bottom that expected progress at the end of YEAR 3 is 2a or 3c.
So in fact by Spring of Y3 - your child has attained the notional target for expected progress.
Yes writing is clearly his weakest subject - but there are ways of cracking that.
Sly encouragement of more writing:
Encourage a diary or designing cartoons.
Encourage your DC to send postcards from places you visit.
Be traditional - and have your DC write thank you cards for presents/ treats from relatives/ etc...
Encourage your DC to enter competitions but to write in (the old fashioned way) and say something nice about the sports hero/ author's books/ etc...
When he has writing assignments from school - look it over - and then discuss with him how he might make it better. I played a game with DD1 (queen of the monosyllable) using Harry Potter characters and having her write sentences describing them.
She started with the deadly boring:
harry potter is a boy
So first off we discussed whether anything might be missing - she agreed a full stop might be nice but I needed to point out Capital H and P for Harry's full name.
They I asked if she could make it more interesting.
She came up with : Harry Potter is a boy and a wizard.
I said good, but I'm still falling asleep. Can you make it exciting and wake me up?
She came up with:
Harry Potter seems like an ordinary boy, but actually he's a wizard.
I then cheered and did a little jig for joy - that is a proper sentence I declared. DD1 got the message. It's now become a bit of a game - I take a peak at her homework now and then and tackle a weak sentence insisting she improve it.
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Have a look at VCOP pyramid - it works top down - such that things at the top are lower ability and things at the bottom show high ability. By using this against what your DC is writing you can get an idea of what he/ she should start moving on to next.
Link here: displays.tpet.co.uk/?resource=387#/ViewResource/id387 - there are 4 sides - so click each little number below image.
HTH