Hi KrazzyMum:
Just a Mum, so not a huge expert - but a lot of what you say sounds just like my DD1 when she was 9.
We could prep for a spelling exam (practicing all week) - she'd ace it or maybe miss 1 or 2, but 2 weeks later - all that work was lost - she'd make lots of mistakes.
At age 7 (end of Y2) she could barely sound out words (very slowly, hugely uncertain of pronunciation), could only add numbers to 20 (because she could count on fingers & toes) and couldn't subtract at all.
Our solution was this:
We determined that for us it was important DD1 could read, could add/ subtract/ multiply/ divide & could reasonably express herself on paper.
We joined an on-line maths tutorial (there's lots out there - just try a search engine) & regularly used it each week
We exchanged books with friends, encouraged all relatives to give books at Birthday/ Christmas & regularly checked out books (because from KS2 guided reading books don't come home & the library at the school had 'issues' with their system so months would go by before they could check out books).
We had DD1 write thank you cards, post cards, old fashioned letters to enter competitions or to tv shows/ characters she liked.
And we chose our battles.
I spent most of Year 3 trying to teach her that with was not spelled whith (she'd grouped it mentally with words like who, what, where, why - which I call question words).
CGP literacy workbooks for Years 3/4/5 are an easy solution (link on amazon: www.amazon.co.uk/KS2-English-Literacy-Workbook-Year/dp/184146158X/ref=sr_1_1?ie=UTF8&qid=1393495968&sr=8-1&keywords=cgp+literacy+year+4 - they were a great help - teaching grammar but also teaching spelling tricks - like 'y' becomes 'i' with endings - so happy becomes happiness or why letters double with endings - so hop becomes hopping to indicate that the 'o' is a short 'o' sound and not the long 'o' in hope - which becomes hoping.
It hasn't totally solved her spelling issues but it really has helped. They were easy to do sort of 10 minutes a week (usually whilst waiting for her sister to finish a ballet lesson) - and gradually seemed to get that message through.
I know it's frustrating for you - and you are ambitious for your DC to do well and thrive - but learning is complicated - some kids learn best through humour/ funny stories/ funny images and others learn through doing. Some teachers explain things really well and others don't. Some classes are quiet and get down to work, and others are full of little whispers and giggles. It's not just down to him, you or the teacher. It really is a complicated mixture of factors - but my advice is doing a bit at home (not heavily but on a sort of scale of 10 minutes here and there - which adds up across a week) - you will slowly start putting some pennies into the bank of learning and gradually see improvements.
HTH