Hi josie14:
First off in my various perusals of education websites I came across this: www.manchesteruniversitypress.co.uk/uploads/docs/610029.pdf
There is a recognised drop in performance in Y3/ Y4 in England.
So this may also be affecting your DS.
I do agree that if you have always been the 'favourite' and suddenly you aren't (something that happened to DD2 - favourite Class R - problematic relationship with teachers Y1 - favourite Y2) this can be really traumatic for children.
Our solution has always been to help where we can at home.
Try exploring BBC Learning [BETA] website: www.bbc.co.uk/schools/teachers/ - in the orange box middle left you can select KS2 and then select the curriculum area you want to explore. There are links to all sorts of activities and if you select options on the far right (once you've entered curriculum topic) you can chose from games, worksheets and activitites.
Try and encourage as much reading as possible. We have bedtime reading just after bath each night. Sometimes they read to us and sometimes we read to them. DH is great at claiming he's a bit drowsy and getting DD1 to read a page of 2 on the sly, to 'give him a rest'. In this way DH and I get the joy of reading our favourite childhood stories: currently DH is reading The Weirdstone of Brisingamen and I read Charlottes Web in Sept/ Oct and A Christmas Carol in late Nov/ early Dec with them. In this way - we all read more, which is great - and DDs see we love it too.
If maths is causing real concern but the issue is speed - I highly recommend Tuxx of Math Command (free to download from here: tux4kids.alioth.debian.org/tuxmath/ - it's like the old game Asteroids and you shoot away problems by typing in the correct answer. Can get a bit stressful but kids generally love it. Also can control skill level and what type of problem you'd like to practice.
If times tables/ division is your concern - I recommend Timez Attack (can download from here: www.bigbrainz.com/. There is a free version or you can pay for more worlds/ more elaborate game playing. We use the free version quite happily though. You are a little boy or girl monster and you run round a dungeon or castle solving multiplication/ division problems. It's quite visual - so you don't just get 4x7 and type the answer - you see four sets of 7 blue balls and count out 7, 14, 21, 28 - then you see 4x7 and can type in 28. Every now and then a giant ogre pops out and quizzes you. Can be a bit stressful - but great fun. They have addition/ subtraction version in Beta out now - but haven't tried it myself and am not certain whether your DS needs this or not.
With writing - encourage sending of postcards, thank you cards and writing off for competitions. Encourage hand-made cards with proper messages (even if only a sentence or two). If this becomes more routine, you gradually find the writing (both patience with writing more than 'happy birthday' and the penmanship gradually will start to show improvement).
You suggest your DS is working at about the level he was at the end of KS1 - so I would recommend trying some of the KS2 bitesize games on the BBC website: www.bbc.co.uk/schools/ks2bitesize/ - you can often chose the level (easy to difficult) and can really work on the elements to be tested at end of KS2.
Anyway I feel for you - I know my DD2 had an awful year last year. The good news is that we're over the 1/2 way point - so just keep telling your DS that really there isn't much longer with this teacher. I know it's hard to find the time for extra support - but 10 - 20 minutes here and there a few times a week really will start to make some differences. The brilliant thing about the games - is you can set them to it and then get on with something else yourself. So you know they're putting in 10 -20 minutes practicing multiplication, but you don't have to do that much at all. Hang in there. It's nearly March and July draws ever closer.
Try your best to focus on the positive and to try and find one good thing in your DS's day at school. The other card to play is when you get that opportunity to talk to a TA/ Teacher your DS had previously, let them know how miserable he is this year. Don't be mean or unfair - just say how sad you are that he's having such a tough time with his teacher this year. You're really struggling to get him enthusiastic about learning/ school. The message will be received and it will make the staff room I can assure you.