Hi shouldbedoingtheironing:
I've posted a lot here about the complete turn around in maths for DD1 we've had by using mathsfactor (www.themathsfactor.com/). It's an on-line tutorial that your DS can work through at his own speed. He'll take a test and they'll start from where he's at. Carol Vorderman (of Countdown fame) used her unfair dismissal money (the fools let her go!) to set this up and it's absolutely brilliant. She uses computer graphics to visually display concepts she's also verbally explainng and slowly, slowly builds skills. There's lots of practice (both gaming and good old fashioned answering sums) and all that practice means that confidence grows.
Now to give equal air time - a lot of other parents have posted on Mumsnet that Maths whizz (www.whizz.com/ and mathletics (www.mathletics.co.uk/) have worked well for them.
Basically - I'd have a look at all three - there are free trials - so have your DS have a play and see which one he likes best. Mathsfactor offers two options: you can make a one-off payment for worksheets (~£7), you can join the summer school (~£15) or you can subscribe monthly to the arithmetic or multiplication school (depending on where your DS is at). We've been doing the arithmetic school for just over 2 years and the change in DD1 is phenomenal. She really understands number patterns, enjoys solving problems and playing with numbers - something I'd have never dreamed possible back in March of Y2 when she couldn't take 1 from 10.
The advantage of going over to something like this is that it removes you from the flash point of teaching your child how to do things your way (which can be stressful for the both of you & often 'isn't the way they do it at school') to just supporting him/ her. You provide a snack, pencil & paper if necessary, answer questions when things (including your computer) aren't working, but basically leave it to the programme. I had to help a lot when DD1 was in Y2/ Y3 (especially with typing) - but gradually over Y4, I've been doing less and less, and really handing over the learning to my DD1. I just am around now, usually getting on with other jobs.
On-line tutorials are great in terms of busy family life - it can fit around school, clubs, play dates, parties, etc... and in terms of mathsfactor (which we use) it is only a time commitment of about 1 hour to 1 1/2 hours a week - but the improvement in DD1's math has been remarkable!
Anyway - I'm just a Mum and I'm sure (as above) they'll be others to come on and suggest x or y programme - but what I'm trying to say is there are things avaiable on-line, which aren't that expensive, that can make a huge difference.
HTH