As always, Gary, you talk a lot of sense, but you miss one point. While schools may not allow colleges to attempt to spirit away their funding, there is presumably no ban on 15/16 year-olds having a look for themselves at what colleges have to offer.
I can't believe that no-one, including the lad's parents, had no idea at all that he might not do very well at maths, so why weren't they looking at the options?
It's too easy to let the school lead the way. Let's face it, your own assertion that five GCSEs ain't up to much is a good indication of how academic you need to be to get a D in maths; it that's you, then get out there and look at the vocational courses on offer elsewhere.
Perhaps this is a chance for KoS to increase its advertising revenue by inviting the colleges to make sure they reach potential pupils in a different way.