I fully understand your problem, but the situation may not be quite as it appears.
Most companies find it very hard to offer work experience, as it does take a reasonable amount of supervision and therefore distracts staff from the paying work. Organisations such as Connexions will promote themselves as the organisers of work experience (because that's how they get funded), and they will certainly do what they can, but they can only work with the companies who are willing to work with them - which tend to be the Tesco's etc who can gaurantee
a reasonable number of places.
However, many smaller companies will take on the occasional work experience student as a favour, but do not wish to be seen as doing so otherwise they get inundated. We're pretty typical, we take on maybe 2-3 of these placements per year, but usually because the student is the child of an employee or of a close friend of an employee. I did take on someone unknown last year, but only because he wrote a blisteringly good application letter and was clearly desperately keen to take up engineering.
SO, unfortunately my best advice is to use any contacts you have, certainly these forums, and those of the other institutes, are a good place to try, but also Googling local IT companies and writing can work, expect to get no response to 99 letters but maybe the hundredth will work! He needs to get an excellent personal statement on one side of A4 which shows his interests and where he wants to go - including in particular anythign he has done in engineering or IT which shows a bit of "get up and go" above the ordinary.
You may have problems with a London based placement as the school normally want to visit the placement to check it's going ok, it's best to discuss this with the school.
I've managed to find excellent placements for my children at the same stage - one in marine biology three years ago and one in music later this year. In both cases I ignored the official systems but instead called in favours from aquaintencies in appropriate organisations. The school was delighted as it saved them the hassle of finding places - and they are still able to handle the H&S paperwork.
Andy Millar CEng MIET CMgr MCMIhttp://www.linkedin.com/in/millarandy
"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert